Astronomy leads us to a unique
event, a universe which was created out of
nothing, one with the very delicate balance
needed to provide exactly the conditions
required to permit life, and one which has an
underlying (one might say "supernatural") plan.
(Nobel Laureate Arno Penzias)1

There has been a bias
among secular astronomers to assume our earth is but one of many
similar planets scattered throughout the galaxy. Indeed, it is
merely an unassuming "speck of dirt" with little to distinguish
itself from the vast number of other similar planets. Carl
Sagan noted that the earth is "a lonely speck in the great
enveloping cosmic dark."2 Furthermore, the fact that life
flourishes on our planet is also ordinary. Creatures of all
forms and types abound through the galaxy, we are told, some of them
probably vastly more intelligent than ourselves. Again, our
planet is nothing very special. Some scientists have estimate there are up to ten trillion advanced civilizations,3 with Sagan
putting the number at one million for our Milky Way galaxy alone.4

The bias toward
intelligent life elsewhere in our galaxy comes from such
considerations.Carl
Sagan and Frank Drake published a now-famous set of assumptions
codified in the Drake
equation.This
formula was based on educated guesses concerning the number of
planets in the galaxy, the percentage of these planets that might
harbor life, and the percentage of planets on which life not only
could exist but which may have advanced to intelligence.In 1974 using the best
assumptions possible at that time, these two astronomers came to the
startling conclusion that life should be common and widespread
throughout our own galaxy.Indeed, they estimated that a million civilizations may exist
in our galaxy alone and since our galaxy is but one of billions of
other galaxies in the universe, the number of intelligent species –
even species vastly more intelligent that us – should be
enormous. Carl Sagan and Drake were so enthralled with the
idea of intelligent life existing "out there" that they beamed a
message of greeting to the great globular cluster M13 which is a
concentration of a quarter million stars in the constellation
Hercules. While it would take about twenty-two thousand years
for this message to reach its destination (and another similar time
frame for any message to be returned to earth), the mere romantic
thought of trying to communicate with civilizations would blot out
those nagging, practical concerns. Carl Sagan was also
instrumental in developing the
massive Search for Extraterrestrial Intelligence (SETI) project that
looks for observable signs of life through the electromagnetic noise
their civilization would admit.It is thought by these
scientists that most intelligent life forms would eventually
discover radio and thereby emit beacons of electromagnetic radiation
into space that inadvertently advertise their existence.Thus far, no such signals
have been found; however, it is assumed that we might not have been
looking in the right spot or just may not have recognized this
beacon.

The current scientific
philosophy states that the forces of nature are so forceful that
life is sure to have evolved on any planet with water if given
sufficient time for natural selection processes to occur.
Whenever scientists raise new speculation about liquid water being
present on another planet - such as on some of Jupiter's moons
(especially Europa and Ganymede) - the automatic assumption is that
there might be some form of life present. Also, if life can
arise with such frequency on foreboding astronomical structures such
as the moons of Jupiter, then why not throughout the universe's
hundreds of billions of galaxies - each galaxy having hundreds of
billions of stars!

David Darling wrote a
book suggesting this optimistic outlook: Life Everywhere.5
He claims that "life may arise inevitably whenever a suitable energy
source, a concentrated supply of organic (carbon-based) material and
water occur together." These ingredients are "starting took
ubiquitous in space" and life should be "widespread."
Furthering this philosophy, Richard Dawkins - the ardent Darwinian
advocate - suggests,

The universe that
we observe has precisely the properties we should expect if
there is, at bottom, no design, no purpose, no evil and no good,
nothing but blind, pitiless indifference.

This is what our
students are being taught in school today - that the earth is
but one of many billions of similar planets, each of which may
well be inhabited by intelligent life-forms, many of which may
well be far more advanced than ours. This is the plot on
such very popular science fiction television shows such as
Star Trek and Star Wars. Even though we do not
at present have a scintilla of evidence that there is any life
outside of our planet, our modern psyche is so affected by this
dominant philosophy that we naturally "know" that life is out
there waiting to be discovered. But the reverse side of
this notion is that we are nothing special; nothing but
accidental "life forms" crawling upon an incidental fleck of
rock orbiting a star that is itself nothing special.

Remarkably, in the
past several years, the entire landscape seems to be changing.
New insights have in a very clear and decisive manner, demonstrated
that perhaps our original understanding of the Universe - and our
planet's position in it - was far too simplistic and simply wrong in
many areas. One researcher noted,

New evidences which
could potentially have refuted the [design] hypothesis has only
ended up confirming it."6

Furthermore, rather
than our lives being meaningless on a fleck of rock, new evidence
makes the remarkable suggestion that we have been placed on this
particular planet to discover and learn about the astronomical
surroundings in which we have been placed in order to perceive that
the universe was created with intelligent design. Furthermore,
claims are now being made in the scientific community that
intelligent life may be extremely rare in the universe - indeed, we
may be the only intelligent life there is!

This provocative
proposal was put forth by geologist Peter D. Ward and astronomer
Donald Brownlee, both professors at the University of Washington in
Seattle, in their best selling book Rare Earth. They
raise th disquieting question about Earth,

What if it is utterly
unique: the only planet with animals in this galaxy or even in the
visible universe...?7

Their conviction that
complex life is "extraordinarily rare" is strengthened by data that
is separate from any theological framework.

Don Johanson, director
of the Institute of Human origins at Arizona State University,
noted,

In spite of our
wishful thinking, there just may not be other Mozarts of Monets.8

David Levy, of comet
Shoemaker-Levy fame, noted,

As we know it on
Earth, complex life might be very rare, and very precious.9

More scientists are
discovering that our planet - against all odds it should be pointed
out - manages to fulfill an extraordinary number of finely balanced
criteria that are absolutely crucial to supporting a habitat
suitable for mankind.

Earth's location, its
size, composition, structure, atmosphere, temperature, internal
dynamics, and its many intricate cycles that are essential for life
(the carbon cycle, the oxygen cycle, the nitrogen cycle, the
phosphorous cycle, the sulfur cycle, the calcium cycle, the sodium
cycle, and so forth) testify to the degree which our planet is
exquisitely and precariously balanced. Frank Press of the
National Academy of Sciences and Raymond Siever of Harvard
University write about what they call "the uniqueness of planet
Earth."10 In this work, they note
how the atmosphere filters out harmful ultraviolet radiation while
storing and redistributing the sun's energy, and how the Earth is
just large enough so that its gravity retains the atmosphere and yet
just small enough so as not to keep too many harmful gases.
They, they describe the Earth's interior as,

a gigantic but
delicately balanced heart engine fueled by radioactivity ... Were it
running more slowly ... the continents might not have evolved to
their present form ... Iron may never have melted and sunk to the
liquid core, and the magnetic field would never have developed ...
If there had been more radioactive fuel, and therefore a faster
running engine, volcanic dust would have blotted out the sun, the
atmosphere would have been oppressively dense, and the surface would
have been racked by daily earthquakes and volcanic explosions.11

This extraordinary and
highly choreographed geological process that produced our planet in
such a way so as to be able to support intelligent life is
astounding and speaks again for the concept of structured
complexity. Michael Denton, a senior research fellow in human
molecular genetics at the University of Otago in New Zealand wrote,

No other theory or
concept ever imagined by man can equal in boldness and audacity this
great claim ... that all the starry heavens, that every species of
life, that every characteristic of reality exists [to create a
livable habitat] for mankind. ... But most remarkably, given its
audacity, it is a claim which is very far from a discredited
prescientific myth. In fact, no observation has ever laid the
presumption to rest. And today, four centuries after the
scientific revolution, the doctrine is again reemerging. In
these last decades of the twentieth century, its credibility is
being enhanced by discoveries in several branches of fundamental
science.12

Astrobiology is the
field of biology that encompasses life beyond our own planet.This new science forces us
to consider the presence of life on our own planet as but a single
example as to how life might work rather than the only
possible example.Astrobiology forces us to consider entire planets as
ecological systems, requiring an understanding of fossil history,
planetary evolution, and geology.New findings from diverse
scientific disciplines are being brought to bear on the central
question of astrobiology; namely, the likelihood of life elsewhere
in the universe.Much
of the revitalization of astrobiology that life on Earth occurs in
much more hostile environments than was previously thought.The discovery that some
microbes could live in searing temperatures and crushing pressures
deep within the sea and deep beneath the surface of our planet led
many to feel that life could similarly exist in the hostile
environments elsewhere in the solar
system.

However, just
knowing that life can stand extreme environmental conditions is not
enough to indicate that life actually exists elsewhere.Not only must life be able
to exist in those environments, but also it must have originated
there.Unless it can be
shown that life can form in hostile environments there is little
hope that life is widespread elsewhere.But biologists counter that
those organisms which do exist in harsh environments on Earth are
genetically the most primitive forms of life indicating to some that
life on Earth may have originated under conditions of great heat,
pressure, and lack of oxygen.
No one has yet developed even a very simple life form from chemicals
in the laboratory using the most sophisticated molecular biology
techniques under very controlled conditions let alone explain how
sophisticated life forms might develop under very harsh early Earth
conditions.

The fossil record is
also revealing.This
record indicates that life originated about as soon as environmental
conditions allowed its survival.Chemical traces in the most
ancient rocks on the Earth’s surface give strong evidence that life
was present nearly 4 billion years ago.This seems to imply that
life forms rather easily and that perhaps life may originate on any
planet as soon as temperatures cool to the point where amino acids
and proteins can form and adhere to one another.Similarly, new ways of more
accurately dating evolutionary advances recognized in the Earth’s
record indicate that animal life – complex life – originated more
suddenly than we previously had expected.Thus, life does not progress
toward complexity in a linear manner but rather in sudden
jumps.The formation of
animal life is much more recent – and much more fragile – than the
formation of early bacterial life.In other words, attaining
complex multicellular life forms is one thing – but maintaining
these forms is quite another.In the case of our own earth, there were countless planetary
disasters that produce mass extinction events that destroy complex
life while sparing the earlier simpler forms.Astrobiologists propose that
on other extrasolar planets, life may form – and perhaps even
complex life in some cases – only to be wiped out by one of these
countless planetary disasters that seem so common.

We have only one
example of a planetary system in which life has evolved; namely, our
own.Our planetary
system is unique in many respects that seem to encourage the
existence of life and promulgation of complex life forms.The earth has been orbiting
a star with a relatively constant energy output for billions of
years.While life may
exist on other planets orbiting other stars, complex life requires
benign conditions that must be stable for great lengths of
time.Animal life
requires oxygen, but it took about 2 billion years for early plant
life to produce enough oxygen to allow early animal life to
exist.Had the Sun’s
energy output varied too much during that long period of
development, there would have been little chance that we would be
here today contemplating life elsewhere.It is difficult, for
example, for complex life to exist on planets in a multiple star
system, or a variable star with its energy unstable energy output
cooking or freezing any primitive life.If life were to evolve on
planets in such an unstable system, it would be difficult for it to
survive for any appreciable time to permit evolution to more complex
life forms.

Not only must the
star around which a planet orbits be stable, the planet itself must
be stable for eons of time.An animal inhabited planet must be a suitable distance from
its star to maintain water in a liquid state, surely a prerequisite
for animal life as we know it.Most planets are either too close or too far from their star
to permit water to exist in a liquid form.Another factor implicated in
the emergence of complex life is our relatively low asteroid or
comet impact rate.The
collision of asteroids and comets with a planet can cause mass
extinctions as we have previously noted.This impact rate is
determined by the amount of material that is left over in a
planetary system after formation of the planets.The most left-over material,
the greater the likelihood of such material crossing planetary
orbits and eventually crashing into the planet with an unbelievable
release of energy frequently sterilizing the planet of any early
life forms.The types
of planets that exist in an early planetary system also determine
the impact rate upon a life-harboring planet.For Earth, there is evidence
that the giant planet
Jupiter acted as a comet catcher – a gravity sink sweeping the
solar system of cosmic debris which otherwise might have impacted
the earth with dire consequences for our own existence.Jupiter may have reduced the
rate of mass extinctions here and may be a prime reason why higher
life was able to form on this planet and then maintain itself long
enough for intelligence to become possible.

The earth is the
only planet (other than Pluto) with a moon
of such appreciable size compared to the planet it orbits, and is
the only planet with plate tectonics which causes the phenomenon of
continental drift.Both of these factors also may be crucial to the emergence of
animal life.

Even the position of
the star
within its galaxy probably plays a significant role in advanced
life development.In
the dense, star packed interiors of galaxies, there are more
frequent supernovae and close encounters with other stars that life
once formed would frequently become extinct.The outer regions of
galaxies may have too little of the heavier elements required to
build rocky planets and fuel the warmth of planetary interiors.Furthermore, our sun and its
planets move through the Milky Way galaxy largely within the plane
of the galaxy as a whole with little movement through the spiral
arms.Even the mass of
a particular galaxy might affect of odds of complex life evolving as
galactic size might affect the odds of complex life developing as
galactic size correlates with its metal content.Some galaxies, therefore,
might be far more amenable toward the development of life than would
others.Finally, our
star – and our solar system in general – are unusual n their high
metal content.Perhaps
even our own galaxy is unusual as well.

Ever since the days
of Copernicus, the earth has been moved from a position of supreme
dominance to its current position of being trivialized.We have become an ordinary
planet orbiting an ordinary star in an ordinary galaxy – a view
formalized by the so-called Principle of Mediocrity.However, it may very well be
that the position of the earth may be much more unique than
previously thought.Indeed, we may be unique.What if the Earth, with its
cargo of advanced, intelligent animals (or so we like to think!) is
virtually unique in this quadrant of the galaxy – the most diverse
planet say, in the nearest 10,000 light years.The Earth may be totally
unique; the only planet with animals in this galaxy or even in the
visible Universe carrying a load of animal life amid a sea of plants
and microbes.When
conceptualized in this manner, the extinction of any animal species
or plant on this planet brought about by the poor stewardship of man
becomes an even greater loss to the
Universe.

Most of the universe
is clearly very inhospitable to life.The vast reaches of empty
space between galaxies and between stars, the interiors of stars,
gas clouds, the surface of gaseous planets such as Jupiter – all of
these locations must be lifeless.While we cannot know for
certain the absolute parameters within which life might exist, we
must assume that it cannot exist under the most extreme
parameters.The Earth
is seemingly an ideal distance from the sun to sustain and nourish
life. The region or distance from the star whereby the formation of
life is considered favorable is defined as the “habitable zone’ or
HZ.The concept of HZ
has been widely accepted and the subject of several major scientific
conferences including one held by Carl Sagan near the end of his
career.

The HZ is the region
where heating from the central star provides a planetary surface
temperature at which water oceans neither freezes nor boils.Our closest neighbors in the
solar system provide mute testimony to what might happen to planets
that are just outside of this HZ.Venus is an example of a
planet that gets too hot; if Venus ever had liquid water it has long
since evaporated and has been lost into space.Alternatively, Mars is
frozen to many kilometers below its surface and are therefore both
outside of the HZ for intelligent life to arise under current
conditions.Similarly,
if the Earth were to become cooler either by the Sun decreasing its
energy output or the Earth to move outward in its orbit, the planet
would soon become ice-covered.Indeed, the Earth has probably become ice covered in its
current orbit in times past.Eventually, carbon dioxide would freeze to form reflective
clouds of “dry ice” particles that would cool the Earth even
further.Eventually,
carbon dioxide would freeze on the polar
caps.

Michael Hart, an
astro-physicist, performed detailed calculations in 1978 to reach
several stunning conclusions regarding the HZ.It is known that the Sun is
becoming slightly brighter over time.About 4 billion years ago,
the Sun was about 30% fainter than at present.Naturally, as the Sun
brightened, the HZ moved progressively outward.Hart then calculated the
small orbital zone wherein the Earth would remain within the HZ over
the entire age of the solar system the “continuously habitable zone,”
or CHZ.His
calculations indicated that the earth would have experienced runaway
glaciation if it had formed 1% farther from the sun and would have
experienced runaway greenhouse heating if it had formed 5% closer to
the sun – both of these effects are considered irreversible.Once totally frozen or
heated, there would have been no turning back.Alternatively, if the
Earth’s orbit had been more elliptical these limits would have been
even smaller.Hart’s
work implied that the CHZ was astonishingly thin for the Sun and
that for stars of smaller mass it did not even exist!These findings suggested
that Earthlike planets with oceans and life were probably rare
indeed.

It is now felt that
Hart’s CHZ is probably too narrow because of several effects which
he did not take into account.One of these is the carbon dioxide-silicate cycle that, on
earth, acts as a regulating thermostat to keep the planetary
temperature within habitable zones.Carbon dioxide is a trace
gas that constitutes only 350 parts per million of the atmosphere
but is very important as a “greenhouse” gas.Carbon dioxide has infrared
absorbing properties that retard the escape of heat from the earth
into space.This
greenhouse effect warms the earth’s surface about 40 degrees C above
the temperature it would otherwise have!As we will see later, the
thermostatic control of the carbon dioxide-silicate cycle occurs
because of the effect of weathering.If a planet warms, increased
weathering removes carbon dioxide from the atmosphere reducing the
greenhouse effect resulting in cooling.When the Earth is too cool,
weathering and carbon dioxide removal decrease, resulting in
build-up of carbon dioxide in the atmosphere and an increased
greenhouse effect leading to warming.This remarkable negative
feedback system widens the CHZ and also complicates efforts to
determine its boundaries precisely.Using this new information,
astrobiologist James Kasting has not defined the HZ as “the region
around a star in which an Earth-like planet (of comparable size) and
having an atmosphere containing nitrogen, water, and carbon dioxide
is climatically suitable for surface-dwelling, water-dependent
life.’In 1993, they
estimated the CHZ as being between 15% greater or 5% less than the
earth’s current orbit – much wider than Hart’s estimate but still
quite narrow – especially for more elliptical
orbits.

The concept of a
habitable zone is now considered more a requirement for advanced,
animal life rather than microbial life.The recent discovery of
extremophiles – bacteria that can live under extreme cold or hot
conditions – requires that the concept of a HZ take on a more
restricted perspective than a few years previously.Furthermore, extremophile
organisms may life under the extreme conditions that exist on other
planets and moons within our own solar system – the most interesting
is Europe – one of the larger moons of Jupiter.This moon is now thought to
probably have a subterranean ocean that might provide a habitat for
extremophile organisms.However, there are two considerations when considering life
under such extreme conditions.First, nothing other than very simple organisms with simple
requirements could exist under such extreme conditions.But most importantly, the
“evolution” of such life forms in the extreme conditions occurring
in such environments would be difficult if not impossible to imagine
at best.

Not only must the
concept of habitable zone be considered in terms of distance, but it
also needs to be considered in terms of time.For example, the earth will
exist in a temporal habitable
zone for approximately 5 to 8 billion years until the Sun’s
continued brightening eventually bakes our planet into conditions
unsuitable for any life.However, for more massive stars, stellar evolution is much
faster.For example,
the life times of stars 50% more massive than the sun would be too
short for the leisurely pace at which life presumably evolved on our
own planet. Stars which are
less massive than the sun (which includes the vast majority of all
stars) burn too slowly and are not as hot as our sun. Planets
would then need to be closer to these stars to sustain life, but
this proximity would make these planets more susceptible to minor
variations in stellar burning rate and tidal forces. Not only
must the star be of the appropriate type to allow for life to
develop, there must also be sufficient time for the elements
making up that life to form. Life requires many
elements that came into existence after the Big Bang (which
essentially produced only helium and hydrogen).Indeed, twenty-six elements
(including carbon, oxygen, nitrogen, phosphorus, potassium, sodium,
iron, and copper) play a significant role in the development of
advanced life, and many others (including the heavy elements such as
uranium) play an important secondary role in supporting life by
creating heat deep within our planet.All of these elements were
created within the centers of star – often in exploding stars or
supernovae – rather than in the Big Bang itself.This means that these
elements were not initially present after the Big Bang, but rather
had to be created within the first generation of stars and so were
not present for perhaps the first 2 billion years or more of the
Universe’s existence.Thus, the habitable zones need to be considered not only in
terms of distance from the central star, but also in terms of time
that such a habitable zone would exist given the known life history
of the central star.

Ejection of Planets out of the Habitable
Zone. Planets might
experience other fates once they start to develop life.Perhaps the most dramatic of
these fates is to be torn from the central star and ejected out of
their solar system and hurtled into the darkness of space.The most common sources of
such ejections are interactions between giant planets.The orbits of the planets in
our solar system have not changed appreciably for billions of years;
however, the planets do interact with each other and the shape of
their orbits do vary somewhat.In general, planetary systems are not stable for the time
period of billions of years for there are interactions between the
larger planets which adversely affect the orbits of smaller planets.
For example, if Saturn were closer to Jupiter or if it were more
massive, then the long-term interaction of these two planets could
lead to ejection of one or the other of these planets causing it to
leave our solar system into the interstellar void.If Saturn were to be lost,
Jupiter would stay trapped in its orbit, but the orbit would become
oddly elliptical.Indeed, some of the large planets orbiting other stars have
highly elliptical orbits and in the distant past ejections of a
long-lost planet might have been the cause.Such ejection of planets
from solar systems is probably even more likely in double or
multiple star systems where the planetary orbits become more
unstable due to the varying gravitational pull of the multiple stars
over the orbital path.

Interestingly,
although a planet ejected from a solar system would have little
ability to sustain life due to the extraordinary cold of
interstellar space, the situation might be different on any moon
orbiting such a planet.Consider, for example, the situation that might arise were
Jupiter ejected from our solar system.Jupiter itself does emit
more energy than it receives from the sun, so Jupiter might indeed
retain some warmth even in interstellar space.But because of the
extraordinary intense gravity of Jupiter upon Europe producing
flexing of the moon generating heat – enough heat to perhaps produce
liquid water deep within its subterranean oceans.Thus, even though Europe
were ejected into interstellar space with its huge planet, the heat
generated by gravitational distortion deep within the moon might
produce enough heat to sustain simple
life.

Habitable Zones in Other Solar Systems. The
brightness of a star determines the location of the habitable zone
but brightness depends upon the star’s type, age, and type.Stars more massive than our
sun have a shorter life along with a more rapid outward migration of
the HZ.For example,
the Sun will be stable for nearly 10 billion years, whereas a star
50% more massive than the Sun enters the giant stage after only 2
billion years.When a
star becomes a red giant, its brightness increases considerable –
maybe on the order of a thousand times – and the HZ increases
greatly beyond its original bounds.Furthermore, stars that have
50% more mass than the sun would not be around long enough for
intelligent life to form at the same rate as it did upon our
Earth.Additionally,
stars that are more massive are much hotter and radiate
substantially more ultraviolet light than the Sun.Ultraviolet light also can
be disastrous for atmosphere such as the Earth as it causes heating
of its upper atmosphere producing progressive loss.Atmospheric loss may prevent
planets similar to our earth from forming oceans and
atmosphere.This
atmosphere problems poses additional problems along with the shorter
lifetime of massive stars making the formation of intelligent life
even more difficult around more massive
planets.

It is often said
that our Sun is a typical star but in fact this is not true.Indeed, 95% of all stars are
less massive than the Sun making our planetary system quite
rare.For the great
majority of stars which are less massive than the Sun, the HZ is
located farther inward. The most common planets in our galaxy are
classified as M stars – they have only 10% of the mass of our
Sun.Such stars are far
less luminous than our Sun, and any planets which might orbit such a
star would have to be located correspondingly closer to the Sun to
stay warm enough to allow the existence of liquid water on the
surface.However, this
closeness of a planet to its star poses additional problems as the
gravitational tidal effects from the star produce synchronous
rotation wherein the planet spins on its axis only once each time it
orbits the star.Thus,
the same side of the planet always faces the star as it orbits; this
is similar to the more familiar tidal effect whereby the moon always
presents its same face to the earth.The result of synchronous
rotation of a planet about a star would produce extreme heat on the
side continuously facing the star and extreme cold on the portion of
the planet always facing away from the
star.

With this
information, we can examine other stars in our Milky Way and
determine whether they might be appropriate places for the formation
of life.Planets
orbiting binary stars (or even worse – multiple star systems) would
be in unstable orbits with extreme variations in local conditions
such as heat and radiation energy.Additionally, a planet
forming in such a system would have to deal with the stellar
evolution of more than one Sun.With two or more suns
undergoing the same evolutionary process, we might expect habitable
zones to migrate and change even faster over time making it even
more difficult for intelligent life to form. This understanding is
extremely relevant as about two-thirds of solar-type stars in the
solar neighborhood are members of binary or multiple star
systems.Other types of
stellar systems would be even less amenable to the formation of
life.Variable stars
(those that exhibit rapidly changing solar energy output) are
certainly poor candidates for producing planets habitable by
animals.Unusual
stellar entities such as neutron stars and white dwarf stars are
probably uninhabitable by any form of life.

Stellar environment
also plays a role in a planet’s ability to sustain life; for
example, in regions where stellar volume is very high, such as open
star clusters and globular clusters.Open clusters are unlikely
to harbor life as they are too young and are composed mostly of
relatively young stars where life would not yet have a chance to
develop.Additionally,
the density of stars in such a cluster would affect the orbits of
each other’s planets to such a degree that it would destabilize
planetary environments making life difficult.Additionally, the large
density of stars in such clusters makes it likely that one of the
stars might explode (become a nova or even a supernova) exposing
nearby space to exceedingly high doses of life sterilizing
radiation.Stellar
density is particular high in globular clusters.These clusters might have as
many as 100,000 stars packed into a space only several light-years
across.The nearest
star (other than our own Sun) to us – Proxima Centauri – is 4.2
light years away.There
are a total of 23 known stars which are 13 light-years away from us,
whereas the M13 globular cluster has 30,000 stars packed into a
space only 28 light-years across!Finally, most globular
clusters are composed of old, heavy-element poor stars, all of which
are about the same age.The low abundance of “heavy elements” such as carbon,
silicon, and iron makes it difficult that any Earth-like planets
could evolve, as they are required for any form of
life.

The understanding
that globular clusters are very poor candidates for containing life
– let along intelligent life – represents a significant advance in
our understanding of the universe.It was only several decades
ago that Drake – one of the authors of the famous Drake equation
that predicted the probability of finding life in the universe –
directed a radio signal toward the globular cluster M13 hoping that
alien radio astronomers in that cluster might receive the message
and respond.Now, we
know that there is no chance anybody will be there to receive the
message when this radio signal arrives at M13 some 24,000 years from
now.

The concept of a
habitable zone can be applied to a galaxy just as it can to
individual stars.Our
own galaxy is a spiral galaxy – the other types being elliptical and
irregular forms).In
most galaxies, the concentrations of stars is highest at the enters
and then gradually decreases toward the peripheral zones.Furthermore, spiral galaxies
are dish-shaped (round, but flat if viewed from the side), with
branching arms when viewed from the top.Our own galaxy has an
estimated diameter of 85,000 light years, and our own Sun is about
25,000 light years from the center.It is located between spiral
arms where stellar density is low compared to the density found
within an arm, and we slowly orbit the center of the galaxy.Our star is located in the
habitable zone of the galaxy.The high density of stars, the probability of dangerous life
sterilizing supernovae and other energy sources, determines the
inner portion of the zone.Alternatively, the abundance of life sustaining heavy
elements determines the location of the outer portion.Outward from the center of
the galaxy, the abundance of elements heavier than helium decreases
substantially.Our
planet has a solid/liquid metal core with some radioactive material
which gives off hear; both attributes may be necessary for the
development of advanced life.The metal core provides a magnetic field that protects the
surface of the planet from significant ionizing radiation from
space, while the radioactive heat from the core, mantle, crust fuels
plate tectonics which is probably necessary for the development of
advanced life.

Newer thought
concerning the concept of a galactic habitable zone has emerged in
recent years.Out
galaxy is composed of four distinct components, each of which
contains distinct solar populations.The region where the sun is
located is known as the thin disk – a flat pancake about 600 light
years thick – inside of which star formation is still quite active,
and young, metal-rich stars are being formed.The second component is the
thick disk which is about three times thicker than the thin disk and
holds older containing fewer metals.The bulge that resides at
the center of the galaxy is the third component, containing a
mixture of both young and new stars.Finally, there is a halo of
spherical star clusters around the disk.These clusters contain the
oldest stars and most metal poor stars in our galaxy with little if
any star producing nebulae.

Star metal content
correlates with position and time within the galaxy.Planets form at the same
time as their parent stars and thus planetary composition also
depend on where and when they form within the galaxy.Old stars formed from
ancient material very poor in metals produce low mass terrestrial
planets.Conversely,
those stars of more recent ancestry formed from relatively metal
rich nebular, should harbor comparatively high-mass terrestrial
planets.

The stars in the
halo and the thick disk are probably simply too old to hold
sufficient quantities of metals for any earthlike planets to
form.The small
terrestrial worlds formed from metal poor nebulae would cool
quickly, develop thick crusts, and lack enough gravity to hold
substantial atmosphere.A thin atmosphere would not be able to shield the planet from
intense ultraviolet radiation.Also, since these small terrestrial planets would tend to
have a thicker crust, plate tectonics would be curtailed and the
climate – whose long-term stability depends upon the removal of
carbon dioxide form the atmosphere by subduction of surface rocks
driven by plate tectonics movement – might become too
unbalanced.Those
planets, which might form near the galactic center, would be exposed
to a greater concentration of stars where they might undergo regular
gravitational perturbations.These perturbations could produce serious impact hazards for
planets orbiting stars that are surrounded by cometary clouds like
our sun.But even more
important, being in an environment filled with other stars
tremendously increases the danger in the form of radiation from
stellar winds, supernovae, or anti-matter and gamma-ray bursts
emitted by unstable stars – or even by the immense black hold at our
galaxy’s center.

This leaves only the
thin disk – but not all of it.As we move along the disk toward the central bulge, star
formation rates increase and metals become correspondingly more
abundant as there are higher rates of star formation and recycling
by nucleosynthesis reactions.These stars would be composed of higher metal contents and
form terrestrial planets that would be too massive.These more massive planets
would have higher concentrations of water and become a water
world.Astronomers feel
that a world that is too wet inhibits the development of complex
life because the interaction of land and sea is a crucial factor in
developing life.

Conversely, as we
move further out into the fringes of the galaxy along the thin disk,
stellar densities drop and planets which might form would have too
little metal content and be too small to support; an
atmosphere.It seems as
though there is a critical distance form the galaxy’s center where
life might form – about 15,000 to 40,000 light
years.

There is some
observational data to suggest the validity of the galactic habitable
zone. These data note that,

All the stars so
far found to have planets, almost none has less than 40% of the
sun’s metal content.

Stars in globular
cluster 47 Tucanae (presumably outside of the galactic habitable
zone) do not appear to have any planets orbiting them.These stars have a metal
content that is about 25% that of the sun and so would not be
expected to have sufficient metal content to form dense
planets.

Not only is the Earth in a rare
position in its galaxy, it may also be fortunate in being in a
spiral rather than in an elliptical or irregular galaxy.Elliptical galaxies are
regions with little dust to encourage new star formation; most of
the stars in elliptical galaxies are first generation stars that are
nearly as old as the universe.The Hubble Space Telescope has also enlarged our
understanding of the early Universe.For ten days during December
1995, the telescope took images of a portion of the Universe only
about 3% as small as the full moon.Examination of these images
was startling – even revolutionary.The images showed galaxies
that were 3 to 15 times fainter than those ever seen before – and
proportionally more distant.There were more than 1500 individual galaxies catalogued in
this very small window of space.These galaxies are so
distant that the light coming from them started its journey long
before our own galaxy even existed, and indeed date to within 1-2
billion years from the Big Bang itself.Thus, stars within these
galaxies do not have earth-like planets as no heavy elements would
have been formed; nor would life be present on any planets which
might orbit these stars as none of the required elements for life
had yet been formed either.

Another insight from
these images was that galaxies formed during the earliest times of
our Universe seemed to have been more irregular than the newer
galaxies.From 30% to
40% of these galaxies are unusual or deformed compared to those
nearer to our own galaxy that were formed much later.As discussed previously,
these irregular galaxies are less likely to encourage life formation
and development, as they are less
stable.

The earth may be
unique among planets in the galaxy and perhaps in the entire
Universe.Additionally,
several of the earth’s neighbors in the solar system have played a
highly significant role in its ability to sustain and nurture
life.The nearly ideal
nature of the Earth as a cradle of life are noted in its prehistory,
its origin, its chemical constitution, and its early evolution.The Earth has the following
characteristics that make life
sustainable,

At least trace
amounts of carbon and other life-forming
elements,

Water on or near
the surface,

Appropriate
atmosphere,

A very long period
of stability during which the mean surface temperature has allowed
liquid water to exist on its surface,

A rich abundance
of heavy elements in its core and throughout the crust and mantle
regions

The
path toward both primitive and then later intelligent life included
element formation in the Big Bang (mostly hydrogen and helium with
trace amounts of lithium) and later in stars, explosion of stars
with the formation of interstellar clouds from which solar systems
and especially the Earth formed.Additionally, the Earth
itself had a complex evolution of its interior, surface, oceans, and
atmosphere.What is
most important to realize is that it is unlikely that the Earth
could be every fully replicated elsewhere in the universe – there
are so many unique characteristics that are all present in our
planet that replication elsewhere seems highly unlikely.While similar planets may
exist elsewhere, it is highly unlikely that everything necessary for
advanced life to flourish is present elsewhere.The factors endless
developmental pathways that led to the formation and development of
the Earth require nearly irreproducible circumstances making similar
development elsewhere statistically impossible.To see why this is possible,
we will start at the beginning by constructing the base elements
required for a planet.

The Earth is very old – over four
billion years.Still,
there was a tremendous history prior to the formation of the planet
for the base elements first needed to be manufactured.It is not considered one of
the most established facts of astronomy that every atom of our
bodies resided inside several different stars before the formation
of our Sun, and has perhaps been part of the bodies of millions of
different organisms since the Earth itself formed.Planets, stars, and
organisms come and go, but the elements which form these objects
have been recycled since the origin of the Universe and are
essentially eternal existing for perhaps 15 billion
years.

All but a very small portion of
the atoms in the planet Earth and in our own bodies were produced
long ago by an intricate set of astrophysical processes.The processes of element
formation were universal and they provided similar starting
materials for other planets existing throughout the solar system,
our galaxy, and the Universe.By looking at this history before our planet formed, we can
gain a certain insight into the range of possible planets and life
habitats that might be present throughout the
Universe.

Everything that
exists throughout the Universe – including time itself – started
with the Big
Bang.While it was
once thought that the Universe is truly eternal with no beginning –
and with no end – it is now certain that there was indeed a
beginning.We go into
why we believe in the Big Bang, and why it is so important to our
theological understanding of God and His plans elsewhere.Suffice it to say here that
there is so much independent evidence pointing toward the Big Bang
that it has become almost universally accepted.In the Big Bang, the entire
Universe was born in an instant in an incredible context of heat and
energy.The subsequence
expansion led to rapid cooling of the primitive universe leading to
the formation of matter out of this tremendous energy utilizing
concepts formalized by Einstein in his famous equation equating
energy and matter.During the first half hour, conditions existed that produced
much of the atoms that are still the major building blocks of stars
– mainly hydrogen and helium.These two atoms make up about 99% of the entire normal
(visible) matter of the Universe.By itself, the Big Bang
produced very little in the way of chemical diversity and gave us
little or nothing beyond very simple atoms; hydrogen, helium, and
lithium.No oxygen,
magnesium, silicon, iron, sulfur, or any of the more familiar
substances that compose 96% of our planet were formed.Nevertheless, the Big Bang
did produce sufficient hydrogen from which all the heavier and more
interesting elements formed.

The temperature of
the Universe during the Big Bang was tremendous – far beyond
anything that can be produced on our planet using the most advanced
technologies we currently possess.During the first half-hour,
the temperature was about 50 million degrees Celsius – at this
temperature positively charged protons (the nuclei of hydrogen)
could occasionally collide with enough energy to overwhelm the
electrostatically repulsive charge of another positively charged
proton to fuse together and form helium.This fusion is the process
by which stars produce energy – and is the reason why the night sky
is not dark.Indeed, it
is the energy produced in the Sun when two protons unite producing
fusion nuclear energy that warms the Earth’s surface.Nuclear fusion energy (as
opposed to nuclear fission energy which powers our current nuclear
reactors) may some day provide virtually endless, clean energy from
water, it may also annihilate all life on earth in hydrogen
bombs.Mankind has yet
to make that decision.

The fusion of two
hydrogen nuclei to form hydrogen was as far as nuclear fusion could
go at the time of the Big Bang.While the temperature was
high enough for helium nuclei to form other elements, the density
was not.The average
density of the very early universe allows us to determine the
ultimate fate of the universe as we discuss elsewhere.Since only hydrogen and
helium were available to the early Universe, planets similar to the
earth could not form.The Universe during the first two billion years of its
existence was unable to have terrestrial planets as it did not have
the elements to produce
them. <![endif]>

Carbon is one of the
most important elements for life, and it could not be formed in the
early Universe because the density of the expanding mass was simply
too low.Carbon
formation had to await the creation of giant red stars where dense
interiors are massive enough to allow such collisions.Because larger stars become
red giants only during the last 10% of their lifetimes (when they
have burnt up most of their hydrogen), there was no carbon in the
universe for the first several billion years after the Big
Bang.Carbon formation
requires three helium atoms to collide as essentially exactly the
same time – a three-way collision.First, two helium atoms
collide to form a beryllium-8 isotope, and then with a tenth of a
femtosecond later (1/10.000.000.000.000.000 second) before the
highly radioactive nucleus dissolves, it must collide with and react
with a third helium atom to produce carbon.A carbon nucleus has six
neutrons and six protons – the combination of three helium nuclei
each of which has two neutrons and two protons.Once carbon is produced in
aging red giant stars, heavier elements come much more easily.The production of heavier
and heavier elements occurs in the cores of stars where temperatures
range from 10 million to 100 million degrees Celsius.The sun, for example, is
currently only producing helium from the fusion of two hydrogen
nuclei.However, during
the last 10% of our sun’s lifetime, it will run out of hydrogen and
start to cool slightly.As the sun cools, gravity will cause the sun to shrink
producing higher temperatures in its core eventually allowing the
nuclear fusion of three helium atoms to form carbon (see above) as
well as other fusion reactions. Eventually, it will produce
all of the elements from helium to bismuth, the heaviest
non-radioactive element in nature.Elements heavier than
bismuth are all radioactive and are produced by the decay of uranium
and thorium.These
radioactive elements will not be produced in our sun because it is
simply not massive enough to produce the heat necessary to create
them.Radioactive
elements are made in the cores of stars ten times more massive than
the sun that undergo supernova explosions – tremendous explosions in
which a star brightens by a factor of 100 billion over the course of
a few days.It is
amazing to think that the uranium that powers our nuclear fission
reactions and which heats the interior of our earth came from the
supernova explosion of a relatively nearby
star.

The elements that
are produced by the Big Bang and in the interiors of stars are the
building blocks necessary for the formation of Earth, any other
terrestrial planet, and life itself.The production of elements
within stars along with continued recycling between stars produced a
relative proportion of these different elements known as the “cosmic
abundance” – the composition of the sun and most common stars.The composition is
approximately 90% hydrogen, 10% helium, with about 0.1% each of
carbon, nitrogen and oxygen, and 0.01% each of magnesium, iron, and
silicon.The Earth
exhibits similar relative abundances of iron, magnesium, and
silicon, some oxygen, but only trace amounts of the other cosmically
abundant elements.

The dominant atoms
that formed the Earth were silicon, magnesium, and iron with
sufficient oxygen to completely oxidize most of the silicon and
magnesium and part of the iron.Other elements have played a
critical role in the emergence of life on Earth despite their
rarity.Carbon is a
trace element in the Earth, but it is a key element for life – and
probably for any alien life as well.Hydrogen is also a rare
element on earth – but it is present in water, the essential fluid
of terrestrial life.The radioactive trace elements uranium and thorium – which
arose in the supernova explosion of stars – heats the earth’s
interior and supplies the furnace that drives volcanism – the
vertical movement of matter within the earth – as well as the drift
of continents on its surface.

The Sun is similar
in composition to other nearby stars but with a major difference; it
contains about 25% more heavy elements than typical nearby stars of
similar mass.Extremely
old stars contain about a thousandth of the concentration of heavy
metals compared to the Sun.The abundance of heavy metals is roughly correlated with a
star’s age.As time
passes, the heavy element content of the Universe gradually
increases so that newly formed stars are on average “enriched” with
more heavy elements than older stars.Furthermore, stars in the
central portion of our galaxy are more enriched with heavy metals
than those stars on the periphery.

The heavy metal
content of stars is important to consider when evaluating whether a
star might have an Earth-like planet.Planets form from a ring of
degree orbiting a star early in its life history.Those stars that have higher
metal contents also have annular rings with higher metal content
that in turn form planets with higher metal contents.If the Earth had formed
around a star with lower, more average metal content, then it would
have been smaller since there would have been less metal from which
it had to form.Being
smaller, the Earth would then have lower gravity and would have
retained less atmosphere, less volcanism, less plate tectonics, and
less of a magnetic field – all of which adversely affect the ability
of a planet to support advanced life as well shall see.Additionally, if the Earth
were further from the center of the galaxy, or even if it were a
typical one-solar mass star, the Earth would probably be
smaller.

Of all the
properties of the Earth, perhaps the least appreciated and the most
curious is that it is so rich in metals.These metals are necessary
to important organic molecules in animals (such as copper, iron, and
magnesium) – but they are so rare elsewhere.How we got such a treasure
of metals is a wonderful story.

Stars are the recycling plants of
the Universe.Over
billions of years, they have taken the raw materials left over from
the Big Bang – namely, hydrogen and helium, and form new matter
through nuclear fusion.Like biological entities, stars are born, live their lives
(only over billions of years), and eventually die.In the process of their
death, they become compact objects such as white dwarfs, neutron
stars, or even black holes.On their evolutionary pathways to these compact entities,
they eject matter back into space where it is recycled and further
enriched with heavy elements.New stars rise from the ashes of older stars and in turn
produce more heavy elements that they in turn eject out into space
as their life ends.That is why each of the individual atoms in Earth and in all
of us have occupied the interior of at least a few different
stars.Just before the
Sun was born, the atoms that would form the Earth and the other
planets in our solar system existed in the form of interstellar dust
and debris.Gradually
over the course of eons, the interstellar matter through
gravitational attraction formed a nebular cloud, which then
eventually further condensed into the Sun, the plants, and their
moons.

The condensation process began
when the mass of interstellar material became dense and cool enough
to gravitationally collapse upon itself and form a flattened,
rotating cloud – the solar nebula.As this nebula gradually
evolved, it quickly assumed the shape of a disk formed ofgas, dust, and rocks orbiting the
proto-sun – a short-lived juvenile state of the Sun when it was
larger, cooler, and less massive as it was still gathering
mass.The planets also
formed within this nebula, even though the nebula itself existed for
only about 10 million years before the majority of its dust either
had formed large bodies or were ejected from the solar
system.

Ground-based
telescopes, and the Hubble Space Telescope orbiting the Earth, have
been able to reveal several lines of evidence suggesting that disks
surround several nearby stars.Among this evidence is a spectacular phenomenon that has only
recently begun to be understood.Young stars show jets of
material radiating out from them.These “bipolar nebulae” are
gaseous objects resembling two giant turnips, each with its apex
pointing toward the star.As stars form, they paradoxically also eject matter back out
into space.The
presence of a disk in the equatorial plane of a star forces the
ejected material into jets along the polar axes of the spinning
system of star and disk.

In our own solar
nebula, 99% of the mass was gas (mostly hydrogen and helium), with
the heavier elements that could exist as solids making up the
remaining 1%.The gas
played a major role in the formation of the gaseous planets Jupiter,
Saturn, Uranus, and Neptune.Dust, rocks, and larger solar bodies separated from the gas
and became highly concentrated, forming a disk-like sheet in the
mid-plane of the solar nebula.

Accretion is the
process responsible for the unique and very important elements of
the Earth.Earth formed
within the habitable zone of the Sun.The paradox of terrestrial
planets is that if they form close enough to the star to be in its
habitable zone, they typically end up with very little water and a
dearth of primary life forming elements such as nitrogen and carbon,
compared to bodies that formed in the outer solar system.In other words, the planets
that are in the right place and have warm surfaces contain only
minor amounts of the right ingredients for life.At the Earth’s distance from
the center of the solar nebula, the temperature was too high for
abundant carbon, nitrogen, or water to be bound to solid materials
that could accrete to form planets.Ice and carbon/nitrogen rich
solids were too volatile (evaporated too easily) and had no means of
efficiently forming solids in the warm inner regions of the
nebula.The Earth has
only trace amounts of these volatile components compared to bodies
that formed farther out from the Sun.For example, carbonaceous
meteorites, thought to be samples of typical asteroids formed
between Mars and Jupiter contain up to 20% water (in hydrous
minerals similar to talc) and up to 4% carbon.The bulk of Earth by
comparison, is only 0.1% water and 0.05% carbon.Had the Earth formed from
materials similar to those found in the asteroid belt farther from
the Sun, its oceans would have been hundreds of miles deep and the
carbon content would have been higher by maybe 1000-fold.However, both the increased
water and increased carbon content would have resulted in a planet
totally covered by water with vast amounts of carbon dioxide in the
atmosphere which would have produced a greenhouse heating
effect.The
resulting temperatures on the surface of the Earth would then be
many hundreds of degrees rather than the balmy temperatures we now
enjoy.The Earth would
have been totally covered with water with only twice as much, and
very few nutrients would have been available in the water to nurture
any life.

The reason why the
Earth is so carbon poor is that much of the carbon in the inner
parts of the solar nebular out of which the Earth formed was in the
form of carbon monoxide gas.If there had been a way to solidify the carbon within this
gas, then carbon would have been the dominant Earth element.A genuinely carbon-rich
planet would be very different form the Earth.The planet would have
graphite on its surface, with diamond and silicon carbide in its
interior.These forms
of carbon would no allow either volcanism or even chemical
weathering, both of which are critically important for animal life
(see below).

The solar nebula out
of which the Earth formed was relatively poor in biogenic elements;
they were present in the outer realms of the nebula.Although much of these
materials stayed in the outer solar system, some would ultimately
have reached Earth by scattering.When they passed near the
outer planets, their orbits would have been significantly altered,
sometimes sending them toward the sun where they might collide with
terrestrial planets such as the Earth.Gravitational effects from
encounters with the larger planets could also cause asteroid and
comet debris, rich in light biogenic elements, to assume earth
crossing orbits.This
“cross-talk” caused some degree of mixing between different zones of
the solar nebula providing a means of bringing the building blocks
of life to what otherwise would have been a lifeless planet lacking
in many biogenic elements because it formed too close to the
Sun.Even today, the
Earth is bombarded by material from the outer solar system in the
form of comets and asteroids.These materials carry carbon, nitrogen, and water – but even
more interesting they also carry organic materials such as amino
acids as were discovered in the Murchison meteorite that fell in
Australia in 1969.

The constant influx
of material to the Earth brings with it life sustaining elements;
however, there is also a darker and more ominous side.The Earth receives about
40,000 tons per year of outer solar material mostly in the form of
small and even microscopic particles.However, occasionally larger
objects also collide with the Earth.An outer solar system object
1 kilometer in diameter randomly strikes the Earth every 300,000
years.Collision of the
Earth with a body this size traveling at a speed of well over 10
kilometers per second results in a very energetic impact event.Every 100 million years on
average, a 10 kilometer diameter objects strikes the Earth.This size object would
produce a rater tens of kilometers deep and over 200 kilometers in
diameter, producing huge fires, and global climate change.Paleontologists believe the
dinosaurs were made extinct by such an impact on Earth 65 million
years ago.Thus, while
debris from the outer portions of the solar system can sustain and
nurture life, it is also capable of destroying life on a vast,
planetary scale.

Larger asteroids and
meteors more frequently struck the early Earth because more were
present.The giant
impacts essentially ended 3.9 billion years ago because other
planets had swept up most of the larger rocks, or they were ejected
from the solar system, or are now present in stable distant
orbits.The continuous
collision of large bodies with the Earth played a role in
determining the initial tilt of the Earth’s spin axis – now at about
23.5 degrees to the vertical, the length of the Earth’s day, the
direction of its spin, and the thermal state of the interior.Furthermore, it is now
believed that the impact of a Mars-sized
body was responsible for the formation of the Moon – a large
satellite relative to the size of its mother
planet.

The final composition of the Earth required several crucial
events

First, there had
to be enough metal present in the earthly Earth to allow for the
formation of a liquid iron and nickel rich core.The rotation of this metal
core permits the formation of a magnetic field – a valuable
property for a planet to sustain life,

Second, there had
to be enough radioactive materials such as uranium and thorium to
enable a prolonged period of heating of the inner regions of the
planet.This
property gave the earth a long-lived furnace that made possible a
prolonged period of mountain building and plate tectonics.Plate tectonics and
volcanism are necessary for maintaining a suitable habitat for
animals as will be seen later on.

Finally, the early
Earth had to have a very thin outer core of low-density material
that also permits plate tectonics to occur – no plate, no
tectonics.

The
production and stability of the Earth’s core, mantle, and crust
could only have come about through the fortuitous assemblage of the
correct elemental building blocks at the right time.

There is no direct
evidence of the composition of the Earth during its earliest periods
as those rocks have not survived.However, it is most likely
that this early period included high-impact collisions which
produced great violence; the greater impacts would have heated and
resurfaced the entire surface of the planet.Some of these impacts might
have vaporized large amounts of water and liberated huge quantities
of carbon dioxide form surface rocks that led to phenomenal
greenhouse effects.The
atmosphere would be heated by retention of infrared energy by the
liberated carbon dioxide producing surface temperatures hot enough
to melt surface rocks.

The final
composition of water and carbon dioxide has had a tremendous
influence upon the Earth’s history as a life-bearing planet.Had the Earth been composed
of just a little bit more water, then there would have been no land
surface; if there had been just a little bit more carbon dioxide,
the Earth would probably have remained too hot to host life – much
like Venus.

The amount and
composition of an atmosphere is of critical importance for the
future life hosting ability of a planet.Today, the composition of
the atmosphere is primarily due to life, and it differs greatly from
those of the other terrestrial planets which range from essentially
no atmosphere (Mercury) to a carbon dioxide atmosphere a hundred
times denser (Venus) to a carbon dioxide atmosphere a hundred times
less dense (Mars).The
Earth’s atmosphere is composed of nitrogen, oxygen, water vapor, and
carbon dioxide (in descending order), and it is not a life that
would be produced by chemistry alone.Indeed, if a space alien
ascertained the composition of the Earth’s atmosphere, he could then
deduce life must be present.Without life, oxygen would rapidly diminish in the
atmosphere.Some oxygen
would oxidize surface materials, while others would react with
nitrogen producing nitrogen dioxide and nitric acid.Similarly, the concentration
of carbon dioxide would probably rist, resulting in the production
of a nitrogen and carbon dioxide
atmosphere <![endif]>

The Earth’s earliest
atmosphere was produced by outgassing of volatile molecules from the
interior.These
volatile molecules were probably carried to the earth by meteors and
impacting comets.The
oceans are a by-product of outgassing and the formation of the
atmosphere.When the
atmosphere was very hot, a great deal of it was composed of
steam.Gradually, as
the Earth cooled down, the steam condensed into water and formed the
vast oceans with which we are familiar.The oceans became salty
through chemical interactions with the Earth’s
crust.

The presence of land
is crucially important for animal life.If the Earth were smooth,
the oceans contain enough water to cover it to a depth of 4,000
meters.Thus, if the
Earth varied by only a few kilometers in elevation it would be
devoid of land.The
formation of land has occurred by two principle means: simple
volcanism creating mountains, and more complex processes related to
plate tectonics.Volcanism leads to the formation of islands such as Hawaii
and the Galapagos archipelagos.Unfortunately, the early
island raising out of the water through volcanism had no life; they
were bleak and desert-like sterile surfaces constantly bombarded by
intense ultraviolet radiation from the Sun unfiltered by the Earth’s
primitive atmosphere.Most of these islands would eventually have collapsed back
into the sea due to constant water erosion form the oceans around
them.Continents were
able to form on the earth that could endure for billions of
years.These continents
required the formation of land masses made of relatively lightweight
materials that could permanently “float” on the denser underlying
mantle.

The early landmasses
may have resulted when the impact of large comets and asteroids
melted an outer region of the Earth to form a “magma ocean” - a
layer of molten rock.Similar processes happened on the Moon, where the magma
oceans cooled, and many small crystals of a mineral called
plagioclase feldspar formed and floated upward to create a
low-density crust nearly 100 kilometers thick.The ancient crust resulting
from these impacts produces the highlands that can be seen with the
on the Moon naked eye.In the case of the Earth, the magma ocean may have led to the
formation of the first continents.The initial landmasses on
the Earth were small, and it was not until half way through its
history land covered more than 10% of the Earth’s surface.The fortunate combination of
surface land surrounded by water was probably very important in
producing a planet that could sustain
life.

The origin of life on this planet is a subject sure to bring
controversy to any discussion.We know more about life now than we have ever known
throughout history; mostly, we know it is more complex and difficult
to understand than we could ever have imagined.For example, last century,
the internal structure of a bacterium was expected to be a
homogeneous mixture or protoplasm without any internal
structure.Today, we
know that the simple bacterium is extraordinarily complicated
producing hundreds of proteins, countless chemical reactions all
tightly controlled by intricate facilitating or inhibitory
reactions.Such
chemical reactions and controlling mechanisms literally cover the
wall of a biochemist’s office.Remember, these are the simplest organisms – the prokaryotes
– and not the vastly more complicated eukaryotic cells.Remember too that this does
not even begin to touch upon the vastly even more complicated
multicellular organisms with specialized tissues all interacting
with other specialized tissues.Life has become vastly more
complicated and even with the explosion of knowledge today regarding
medicine and the treatment of disease, we are only just beginning to
understand the biochemistry of life.

One of the assumptions held by those who promote naturalistic
evolution is that there was plenty of time for the emergence and
evolution of life; after all, there was about 3.5 billion years
since the emergence of bacteria until today.However, discoveries about
the universe and the solar system have shattered that
assumption.What we now
realize is that life originated on Earth very quickly.Fully formed cells first
appear in the fossil records as far back as 3.5 billion years ago,
and limestone rock which was formed from the remains of organisms,
dates back 3.8 billion years.The ratio of Carbon-12 to Carbon-13 found in ancient
sedimentary rock also indicates a plentitude of life on Earth for
the era of about 3.8 to 3.5 billion years ago.From 4.25 until 3.8 billion
years ago, the bombardment of Earth was so intense that no life
could possibly exist.From 3.8 until 3.5 million years ago, the bombardment
gradually decreased to a comparatively low level; however, it has
been estimated that during those 300 million years at least thirty
life extinguishing impacts must have occurred.The importance of these
facts is that life sprang up on Earth in what could be called
geologic instants, periods of ten-million years or less (between the
devastating impacts).Thus, we do not indeed have the billions of years for life to
occur on this planet; rather, we have perhaps only several million
years.

Evolutionary theory
posits that life came forth spontaneously out of chemical reactions
aided by heat and perhaps electricity in a “prebiotic soup” of
chemicals.However,
attempts in the laboratory to demonstrate that life can and does
spontaneously come together on its own have resulted in
failure.Even under
highly favorable conditions of a laboratory, these soups have failed
to produce anything even remotely resembling life.One problem is that they
produce only a random distribution of left and right-sided prebiotic
molecules, whereas life chemistry demands that all of the molecules
be eight right or left handed.With all our learning and technology we cannot even come
close to bringing life together in the highly controlled environment
of the laboratory.How
can we expect life to arise spontaneously in the chaotic environment
of an ancient earth?

Other evolutionists recognizing these difficulties have
posited that life came from extraterrestrial bombardment.Forgetting about the
destructive nature of these planetary collisions, they hypothesize a
possibly beneficial effect.It is proposed that perhaps this bombardment may have
assisted the formation of life by delivering concentrated doses of
prebiotic molecules.Comets, meteorites, and space dust in general are partly
composed of carbon and interplanetary dust particles can carry some
prebiotic molecules, they carry far too few to make a real
difference.In fact,
with every potentially helpful molecule that they might bring to the
planet, they bring several more that would get in the way – useless
molecules that would substitute for the needed ones – again, that
left and right-handed problem again.

Carl Sagan – one of the great scientists in the evolutionary
movement (and who has just recently died from cancer), clung on to
yet another chance whereby life might come to this earth.He suggested that perhaps
the atmospheric conditions 3.8 billion years ago when life first
appeared were not too unfavorable for life – perhaps the conditions
were just neutral.An
“oxidizing” atmosphere would be unfavorable as the atoms and
molecules would bond with oxygen and removed from being used for
life molecules.Alternatively, a favorable atmosphere would be one which
would be a “reducing” atmosphere in which molecules bind with
hydrogen rather than oxygen.But even these tentative hopes were dashed about five years
ago when atmospheric scientists established that the Earth’s
atmosphere has been fully oxidizing for at least the past four
billion years.Under
these conditions, processes producing amino acids (which build into
proteins) and nucleotides (which are used to produce DNA and RNA)
would operate 30 million times less efficiently than they would
under reducing conditions.Natural primordial soups would thereby contain far too few
prebiotic molecules to overcome this inefficiency – not to mention
the destructive chemical reactions would be tremendously
increased.Finally, the
small amount of amino acids that would be produced would consist
mostly of glycine – vanishingly little of the other amino acids
would be
produced.

Some have proposed that perhaps we are expecting too much and
the life that was formed spontaneously 3.8 billion years ago was
much simpler than that which exists today.However, the minimum
complexity of an organism has to be such that it can reproduce
independently, and the simplest life forms currently in existence
today is about at that level.Furthermore, if very simple life were able to be produced in
a prebiotic soup, then that very simple life should be easy to
create in the laboratory.Such however, is not the case; indeed, as has been previously
noted, nothing resembling a biologic life form has thus far been
created.

Several papers published in the prestigious journal
Science recently gave some hope to evolutionists by proposing
what seemed to be a possible way around some of the complications of
the complexities of life.For life to occur, you need DNA that holds the blueprint for
life, RNA which are molecules that carry information form the DNA
molecule to specific proteins, and then proteins themselves.Furthermore, any one of
these three basic ingredients is insufficient by themselves; rather,
you need all three of them together spontaneously.Even the most optimistic
researcher would agree that the chance appearance of these
incredibly complex molecules at exactly the right time and place for
the initial production of life was beyond the realm of natural
possibility.

However, in 1987 a
research group demonstrated that one kind of RNA can act as an
enzyme or catalyst, and investigators have proposed that perhaps the
earliest life form utilized a chemical that acted like DNA, RNA, and
protein – performing all three functions.However, this still does not
help the evolutionist at all because this purported intermediate
chemical would still have to carry all the information of those
chemicals it is replacing.In other words, the task of assembling such an incredibly
versatile molecule (which has not been synthesized in the
laboratory) would be as difficult as assembling the three different
kinds of molecules it would be replacing.

The catch in these notions is that RNA is more easily
assembled that DNA.Indeed, for twenty years, it was taught in evolution
textbooks that RNA had been synthesized under prebiotic
conditions.However,
this myth was exploded by Robert Shapiro in a meeting of the
International Society for the Study of the Origin of Life held at
Berkeley in 1986.Furthermore, Shapiro demonstrated how the synthesis of RNA
under prebiotic conditions is essentially impossible.Shapiro then went on to
publish his research, an his proposals remain unchallenged to this
day.

Humans constantly
make errors; we live in an imperfect world and there are errors
constantly occurring with our genetic material.It is now thought that some
of these errors may be the primary cause for cancer and for many
other chronic illnesses.Furthermore, radiation causes errors to occur in the genetic
material – but radiation is important in the stability and life
sustaining capacity of this Earth.Therefore, it is important
that life molecules be so designed so that they can repair errors
which might happen within them over time.A useful analogy for
visualizing the error handling capacity of the genetic material is
to consider a computer program with a few million lines of
code.In spite of
random destruction of ten thousand lines of code, the program still
performs its intended function.Up until now, believe me –
nobody has written such code but such a code exists in the genetic
material.There are
many areas in the genetic code where random destruction or changes
would have little or no effect upon the protein produced.Life molecules are designed
so that they can function even after limited, random
destruction.

The majority of
astrobiologists believe that the temperature of the Earth from the
time of the emergency of simplest life – about 3.8 billion years ago
– until the origin of more complicated organisms – about 2.5 billion
years ago – was high.The concentration of oxygen in the early atmosphere was
vanishingly low, far too low to support animal life.Gradually, the greenhouse
gases in the atmosphere dissipated and the Earth’s temperature
declined.There is
evidence to suggest that there have been as many as four major
episodes of glaciation on the Earth on a scale far exceeding
anything we might imagine.

The first Snowball
Earth period began about 2.45 billion years ago, and a second siege
of several such glaciations occurred between 800 and 600 million
years ago.These two
snowball periods are of great interest to astrobiologists because
they are signal events in biological history.Around 2.5 billion years ago
around the first great glaciation, the first eukaryotic – or animal
like - cells appeared.Furthermore, the fossil records indicate that about 550
million years ago, diverse and abundant animal life blossomed to
such a great extent that it is known as the Cambrian Explosion.These two explosions of new
life forms occurred immediately after the two most severe episodes
of glaciation and ice cover in the Earth’s history.

Trillites are
deposits of angular rock fragments that glaciers deposit as they
move across the landscape.Recent ice ages have left many of such deposits in the
Northern and Southern hemispheres.Interestingly, these
deposits are recovered from virtually all latitudinal regions of the
globe which shows that the glaciations must have extended to near
equatorial latitudes.These “snowball” glaciations were much greater than the
recent Ice Age in which glaciers only extended to
mid-latitudes.The
“Snowball” Earth periods were times when the planet teetered
dangerously close to becoming too cold for any life to survive.The Snowball Earth theory
received greater status in astrobiology circles when Harvard
geologist Paul Hoffman’s study, published in a 1998 issue of
Science, demonstrated that ice extended to near equatorial
latitudes in the late Precambrian Era – about 700 million years
ago.What has been
recognized is that unlike the more recent Ice Ages, the Snow Ball
earth all of the oceans (in addition to the land) was covered with
ice to considerable depths.Only the deepest portions of the oceans remained liquid while
the ocean was covered with ice to a depth of perhaps 1500
meters.During these
times, the Earth would have been extraordinarily cold with the
average surface temperatures varying between –20 and –50 degrees
C.

These extremely cold
temperatures would have had a tremendous influence upon the surface
of our planet.For
example, continental weathering would have been slowed or even
stopped; there would have been no or little exposure of land to the
elements for weathering to occur.Furthermore, the presence of
an ice cover over the oceans would have acted as a lid separating
water from the atmosphere.Little water could evaporate out of the oceans for water
would have been uncoupled from the atmosphere.Volcanism would have
continued, but there could be less release of toxic gases into the
atmosphere and more into the ocean from underwater volcanic
activity.Instead,
these gases would become dissolved in the water and created a very
toxic environment indeed.These condition would continue for extended periods of time
during a Snowball Earth – as long as 30 millions
years.

Astronomers
previously had believed that such an “icehouse” or “snowball” earth
would be irreversible reasoning that as a planet gets more and more
thickly covered with ice, the fraction of light reflected back into
space increases and solar heating of the surface of the planet
decreases.A planet
covered with ice would reflect most sunlight back into space causing
the planet to become even cooler. Yet, it appears as though the
Earth was able to escape the grip of a deep freeze several
times.The means of
this escape is thought to be through volcanism whereby greenhouse
gases such as carbon dioxide are released into the atmosphere
producing a “greenhouse effect” that would tend to absorb and retain
heat.Gradually, the
oceans would again melt and the Earth would undergo spectacular
changes.Kirschvink
described these events as follows,

“Escape from
this “icehouse” condition was only accomplished by the buildup of
volcanic gases, particularly carbon dioxide, mostly from undersea
volcanic activity.Deglaciation during the end of these glacial events must
have been spectacular, with nearly 30 million years of carbon
dioxide, ferrous iron, and long buried nutrients suddenly being
exposed to fresh air and sunlight.Hundreds of meters of
carbonate rock are preserved capping the glacial sediments, at all
latitudes, on all continents, as a result of wild photosynthetic
activity.For a brief
time, the Earth’s oceans would have been as green as Irish clover,
and the sudden oxygen spikes may have sparked early animal
evolution."

Phytoplankton represents the most important source of
biological productivity in the oceans.These tiny one-celled plants
produce considerable amounts of oxygen and are responsible for a
large portion of the oxygen we animals use every day.The growth of these plants
is limited by the availability of nutrients and iron.If iron is dropped into the
oceans of today, a great bloom of phytoplankton will result.Soon after the end of the
snowball earth, the stored iron and magnesium in the oceans would
have acted as fertilizer tremendously stimulating growth of the
blue-green “algae” – really photosynthesizing bacteria known as
cyanobacteria.Huge
amounts of oxygen would have been released by the ocean bloom that
preceded the appearance of new life.

These events would have tremendous effects upon the Earth’s
geology as well.The
sudden rush of oxygen produced by plankton into the sea would have
caused the iron and manganese rich oceans to precipitate out iron
and manganese oxides.Evidence of this precipitation is seen in South Africa where
the world’s largest land based deposit of manganese minerals has
been dated at 2.4 billion years old.This deposit sits on top of
sedimentary deposits that were laid down during the 2.5 billion year
old first snowball Earth, and appear to be a direct consequence of
the oxygen bloom that occurred when this snowball
melted.

The oxygen that was released into the atmosphere initially
was a poison to many forms of life.Having developed during a
time when there was little or no oxygen in the atmosphere, the
sudden appearance of oxygen would have been a disaster.However, the newfound oxygen
apparently stimulated some life forms and for them it was a powerful
spur toward further growth and development.Organisms had only one of
two possibilities left to them; die, or adapt to the newly produced
oxygen flooding the planet.Organisms would have to develop two major
adaptations.

First, enzymes needed to
develop which would help the cell avoid the ravages of dissolved
oxygen and chemicals called hydroxyl radicals. These
chemical oxidize critical components of cellular structure and
over the period of years can seriously damage an organism.
Today, mankind is still trying to develop means for neutralizing
these oxygen radicals for they are felt to be a major factor in
the development of dementia, heart disease, and even certain forms
of lung disease. We take vitamins - such as Vitamin C and E
- to reduce the effect of these compounds.

Second, the huge amount of
oxygen liberated by the growth of plankton and other plants
oxidized iron and other metals dissolved in water making them
harder to nourish life. After having been surrounded by
high-iron solution since the first formation of life, proteins
within cells had to be reengineered for life in an environement
which was iron poor.

DNA sequencing
techniques have shown that several enzymes found in ancient bacteria
are left over from this event which occurred some 2.5 billion years
ago – no such enzymes occur in more ancient bacteria.The implications of this are
profound suggesting that the oldest bacteria developed before the
Snowball Earth as they do not have the enzymes required to deal with
dissolved oxygen radicals or iron rarity.Rather, the Archeae and
Eucarya developed after this seminal event as they do have the
required enzymes. Indeed, the record of the oldest eucaryan – a
creature discovered in 1992 known as Grypania – are found in rocks
about 2.1 billion years old – far older than the bacteria which
preceded them.The
Grypania are the oldest organisms that had attained the eukaryotic
style of organization with internal organelles and
structure.They are
found in iron deposits located in Michigan and are in chains as much
as 90 millimeters long.This discovery indicates that the first eukaryotic cell
occurred during the banded-iron formation process when there was
little free oxygen in the sea and atmosphere.The Grypania may have been
very rare as other eukaryotes do not occur in the fossil record for
another 500 million years.

Thus, the original Snowball earth that occurred about 2.5
billion years ago was extremely important for the development of
intelligent life for the following
reasons,

First, the Snowball produced the largest mass extinction in
our planet’s history due to extremely cold temperatures which
existed down to the equator regions of the earth.All water was removed from
the surfaces of continents due to the extreme
cold.

Second, the Earth’s release from the Snowball effect would
bring about a new catastrophe as nutrient rich water became
exposed to warmth and plankton life bloomed releasing vast amounts
of oxygen into the atmosphere.For most bacteria and
simple plants, this oxygen would prove toxic and produce immediate
death.Some life
managed to survive by developing means to deal with the oxygen and
thrive; this life would then expand to fill the ecological niches
left from that life which did not
survive.

Third, the first eukaryotic cells were produced about 2.2
billion years ago, laying the foundation for more complicated,
diverse eukaryotic cells that would form about 1.6 billion years
ago.

The Second Global Glaciation

Another great period of life extinction occurred about 800
million years ago.As
with the earlier deep freeze, the earth was once again locked into a
global icehouse.All
life had to retreat to sources of heat such as around volcanoes and
hydrothermal vents – or die.After the first great snowball earth, there was a great
increase in the diversity of bacterial, single celled life with the
emergence of eukaryotic life – such as the cells which make up our
body.The second global
glaciation similarly produced mass extinction of most life on earth
– all life that could not somehow escape the cold was
destroyed.Both of the
two major episodes of Snowball Earth nearly ended all life on the
planet, but each ultimately opened up new opportunities for life to
develop.The end of the
last Snowball Earth event ended the Precambrian era; soon
thereafter, abundant skeletons of larger animals begin to fill the
sea.New life forms
suddenly appeared in what has been called the Cambrian Explosion –
an “explosion” of new life.

The Earth’s surface temperature seems to have a determinant
effect upon the form of life which might develop.Simpler organisms such as
the bacteria can have very large tolerances for temperature
variations.Indeed,
most bacteria can live in temperatures ranging from the freezing
point of water to the boiling point – and some can survive in even
greater temperatures (the methanogens and thermophiles).However, more complex cells
such as the eucharyotic cells which make up our body, have much
narrow temperature tolerances.Eucharyotic cells have multiple small organs (organelles)
which life within them such as the mitochondrion.This organelle is
responsible for producing energy for the cell and is therefore
critically important.However, the mitochondria have an upper temperature tolerance
of about 60 degrees C, much lower than the boiling point of water
(100 degrees C).

Earth was without
life for the first 3.5 billion years of its existence, and was
without animals that were large enough to leave a visible fossil
record for nearly 4 billion years.But about 550 million years
ago, there was a sizable and diverse animal life that burst into the
oceans – the Cambrian Explosion.Over a relatively short
period, all of the animal phyla either evolved or first appear in
the fossil record.No
matter where on Earth we look, there are no skeletons of animals in
rocks older than 600 million years! Yet, these animals are plentiful
and diverse in rocks dating from 500 million years ago indicating
that they all must have developed during this relatively short
period of 100 million years.Thus, it would seem that our planet went from a place without
animals that could be seen with the unaided eye to a planet teeming
with invertebrate marine life equal in size to that observed
today!

The
rate of production of new species has never been equaled since the
Cambrian Explosion.Prior to this event, there were a very few species of animal,
each growing to only a very small size.During the Explosion,
however, there were huge numbers of new species many with completely
new and previously unseen body parts!

Trilobites were ancient animals that came forth during the
early portions of the Cambrian Explosion.They look like nothing that
is alive today although they resemble the horseshoe crabs and pill
bugs.The trilobite
fossils range in size from the microscope to nearly 3 feet in
length.They have
numerous spines, great helmet-like heads, and a variety of peculiar
eyes.They are quite
complicated fossils left over from complicated creatures.At many localities
throughout the world with sedimentary rocks of approximately 550
million years of age, the first obvious fossils are trilobites
perched on top of thick sequences of rock apparently devoid of
fossils.This
observation seems to indicate that complex life appeared upon the
earth without precursors; as though the orchestra began to play
without any warm up.

An English geologist
Adam Sedgewick is credited with naming the geologic unit of time the
Cambrian era, defined by a thick layer of sedimentary rock found in
Wales which contained a characteristic group of fossils including
the trilobites.With
modern dating techniques, this layer of Cambrian era sedimentary
rock containing evidence of the Cambrian explosion is dated between
540 and 490 million years ago.The earliest portions of Cambrian era rock is defined as that
portion where the first trilobite fossils could be found – not only
in Wales – but throughout the world.This definition
prevailed for over a hundred years and has only recently been
changed.Today, the
start of the Cambrian era is where the first trace fossil - the
fossilized record of animal behavior rather than the animal itself –
is located.The
Cambrian explosion has certainly been a difficult observation for
evolutionists to explain, and was especially worrisome for Charles
Darwin.In his seminal
work, On the Origin of Species, Darwin speculated that the
era before the Cambrian era must have “swarmed with living
creatures.” However – where were the fossils of these “swarms?”If Darwin were correct, then
a long period of evolutionary change with simpler precursors should
be evident in the fossils to eventually through evolutionary change
produce the complex creatures collected by Sedgewick.Darwin was never able to
refute this most stringent criticism of his theory, but instead
rallied against the imperfections of the fossil record, believing
that there must be a missing interval of strata just beneath the
trilobite layer.

Darwin was at least
partially right, but science had to develop further.Modern radiometric dating
techniques now date the Precambrian/Cambrian boundary to 543 million
years ago.Interestingly, the “Middle Cambrian is dated at 510 million
years ago, whereas the oldest trilobites are no more than 522
million years old.Therefore, the bulk of Cambrian time was
“pre-trilobite.”It was
in this pre-trilobite era that animals developed which were tiny and
lacked skeletons so they rarely left obvious traces in the fossil
record.They are indeed
hard to detect unless special processing techniques are used to
extract them from their entombing matrix.But, what we do see is the
rapid, amazing development of complicated, animal life with fully
developed skeletons developing from unicellular life over the time
period of only about 12 million years – a phenomenally short time as
even the staunchest evolutionists admit

There were three major jumps in complexity which occurred in
relatively short time with few if any precursors; seemingly new
creations.These three
major jumps are the formation of the first eukaryotic cell, the
first multicellular organism, and the Cambrian explosion of
life.

The first life forms appeared on this Earth about 3.5 billion
years ago as the bacteria.From what we can tell from the records these ancient life
forms have left behind, they are structurally very similar to
bacteria in existence today.Their biochemistry his now more diversified as some are more
specialized to live in environments without oxygen while others can
only life in an environment rich in oxygen; some might live better
in cold environments while others crave warmth.However, the basic structure
of bacteria has remained essentially unchanged over the billions of
years since their first appearance.

Eukaryotic cells then burst on the horizon about 1.5 billion
years after the first bacteria.While the simple bacteria
(prokaryotic cells) did not change in form over the billions of
years since their appearance, exactly the opposite is true
concerning the eukaryotic cells.The eukaryotic cell is the
basic prerequisite for the formation of complex metazoan
(multicellular) life from which arose more complicated life forms
including plants, fungi, various protists forms (protozoans,
ciliates), and ultimately animals life dogs, cats, and finally
humans.The eukaryotic
cell is fundamentally different from the earlier prokaryotic
cells.Evolutionary
biology has a difficult time accounting for the arrival of this new
life form as different from the prokaryotic cell as a human is from
a sponge.There are no
intermediaries seen in the fossil record; rather, this fundamentally
new life form suddenly appears.

The eukaryotic cell has
seven major characteristics that differentiate them from their more
primitive cousin – the prokaryote,

Eukaryotes have more flexible walls that allows them to
engulf food particles through a process known as
phagocytosis

Eukaryotes have an internal structure system composed of
tiny protein threads that allow them to control the location of
internal organ systems.This structural system also helps the cell duplicate their
DNA into two identical copies during cellular division.This process is more
precise than the simple splitting of DNA that occurs in the
prokaryote cell

Eukaryote cells are much larger than prokaryotes; they
usually have cell volumes that are at least 10,000 times that of
the average prokaryotic cell.The eukaryotic cell has an
internal architecture and salt balance regulating systems that are
far more advanced and precise than that found in the prokaryotic
cell making this larger structure
possible.

Eukaryotic cells have much more DNA than prokaryotic cells
– usually 1000 times as much!Furthermore, the DNA found
in eukaryotic cells is stored in strands or chromosomes, and it is
found in multiple copies in every cell,

Eukaryotes have many
other enclosed organs within their cells including the
mitochondria (which produce energy), and chloroplasts (with
chlorophyll to produce energy in plants)

In eukaryotes, DNA is contained within a membrane-bound
organelle, the nucleus,

The
first multicellular organisms were most likely formed from
prokaryotic cells.Cellular slime molds are multicellular, as are some
cyanobacteria.However,
development of these prokaryotic cell forms did not seem to advance
any further; this was an evolutionary dead end.These multicellular life
forms have existed on the Earth for billions of years with little if
any further development or differentiation.

The
first jump from single-celled organisms to multiple celled organisms
seems to also have happened suddenly.Single celled eukaryotic
organisms have a cell wall which helps to insulate them against the
environment.This cell
wall was shed in order for cells to aggregate and communicate
amongst themselves.Furthermore, cellular differentiation had to occur, such that
some cells might become gut cells, while others carry on nervous
system functions.Also,
while there are a few very simple animals with only two tissue type
plan (ectoderm and endoderm), most multicellular animals have three
tissue types – ectoderm, endoderm, and mesoderm.Finally, these multicellular
animals had to develop a means of reproduction, locomotion,
nutrition, and defense.

The
largest and most primitive of higher eukaryotic organisms are the
sponges.These
creatures have different cell types which perform specialized tasks,
but there is very little in the way of communication and interaction
among cells.There is
no intestinal cavity for processing food, and there is no nervous
system.New organisms
with similar structure, and others with vastly more complex
structures sprang suddenly into existence at the time of the
Cambrian explosion.There were at least 50 to75 new phyla or large groupings of
animals that suddenly appeared; many of which have since become
extinct; however, since this Cambrian time period, no new phyla have
come forth.It is as if
the creative processes were briefly turned on to produce vast new
and complex forms of life in the period of only several million
years only to be suddenly turned off so that no new animal phyla are
created for the next several hundred million years.Scientists perhaps for
obvious personal bias primarily study animal life and development,
but similar sudden creations of new plant phyla also appeared but
are much less studied.

Naturally, the problem of the Cambrian explosion poses a
difficult challenge to conventional evolutionary theory on several
levels.

First, there are no intermediate forms showing, for
example, how a sponge of mollusk might develop into a “higher”
animal such as a fish.The problem of intermediate forms is present for the entire
evolutionary theory as a whole – not just for the Cambrian
explosion.

Next, it is difficult to
explain (although attempts have been made (see below) how
evolution was briefly turned on to such an extent as to create
vast quantities of hugely different organisms – only to be turned
off again.

Finally, the essential problem of
evolution – exactly how macroevolution occurs – is still
present. It is one thing to say that there were factors
favorable for life shortly after a snowball earth or during time
periods when new landforms arise, but it is quite another to
develop a theory as to exactly how macroevolution occurs. In
other words, how an organism suddenly develops vastly new
structural characteristics that are fine-tuned to the organism
which have never been seen before has never been satisfactorily
addressed.

Most of the proposed explanations for the vast explosion of
new life forms during the Cambrian time epoch concern environmental
changes that also seem to be occurring. One of the most
important physical changes that was occurring on the earth was in
the levels of oxygen. It is proposed animal life was
encouraged by this increase in oxygen levels throughout the
planet.

The current oxygen rich atmosphere is a late occurrence on
this planet. For much of the Earth’s existence, the level of
oxygen would not have supported animal life as we know it –
certainly not the fast moving animals with which we are most
familiar. Modern animals use large volumes of oxygen to move
their bodies quickly across land; such would be difficult or
impossible with an anaerobic (oxygen lacking) respiratory
system. Humans use about 250 cc of oxygen every minute at rest
– much more with exercise – without which we would quickly cease to
function.

Bacteria seem to be the primary cause for the earth’s oxygen
rich atmosphere with the aid of multiple other factors.
Cyanobacteria are those which produce oxygen as a waste product but
only in minute amounts. Early in the Earth’s history, most of
the oxygen they produced was absorbed quickly by the Earth’s crust
and mantle as metal oxides were formed. These reducing
minerals acted as a sponge soaking up much of the oxygen produced
over countless millions of years. With tectonic activity, new
portions of the Earth’s mantle was repeatedly exposed to the
atmosphere sucking up vast quantities of oxygen while oxygen rich
exposed portions of the mantle were submerged below the Earth’s
surface. Over time, the quantity of reducing materials in the
mantle slowly diminished and less oxygen was absorbed.

Models indicate that between 3.9 billion and 2.7 billion
years ago, reducing minerals in the mantle and crust efficiently
absorbed most of the oxygen produced by the cyanobacteria.
However, between 2.7 and 2.2 billion years ago, gases released from
volcanic activity had lost much of their oxygen absorbing ability,
and from 2.2 years ago until the emergency of large quantities of
animals during the Cambrian explosion, the atmospheric oxygen
concentration slowly increased. One of the most important
aspects of this oxygen production was the removal of methane from
the Earth’s atmosphere.Methane is a gas (“swamp gas”) produced by decaying material
among other causes. Even in small quantities, methane is one
of the greenhouse gases (such as carbon dioxide) and effectively
traps heat into the Earth’s atmosphere.As the increasing oxygen
levels removed more and more methane, the earth cooled and went into
several periods of glaciation, the last one occurring just before
the Cambrian explosion.

It is presumed by evolutionists that perhaps one stimulus to
advanced animal life was the rising level of oxygen in the Earth’s
atmosphere perhaps giving them an evolutionary survival
advantage. However, the precise mechanism underlying the
differentiation and production of so much new life merely by
increasing oxygen levels has not yet been defined but remains only a
theory – a very speculative theory – at best

Biological Changes. Another mechanism
proposed by evolutions to explain the Cambrian explosion is that the
biological changes were themselves triggering some of the physical
events. In this scenario, the common use of calcium carbonate
shells by newly evolved animals changed the way calcium was
distributed in the oceans. Similarly, organisms may have
favored the formation of phosphorus and not the other way
around.

Nutrient Availability. Abundant evidence
suggests that at the end of the last Precambrian era there was a
relatively sudden and dramatic increase in the amount of available
nutrients. It has been proposed by some that these nutrients
placed a great pressure on the production of more life that also
somehow increased evolutionary pressure. While this may
initially seem plausible, upon closer consideration several problems
arise. First, the problem of producing completely new and
different animal life is not explained by adding more nutrients to a
population! Indeed, the opposite would seem to be the case for
more nutrient production would favor the survival of weaker, less
adaptive creatures rather than encouraging the development of new
and different creatures.

Cambrian
Cessation

The Cambrian explosion has
long been explained, as noted above, as an explosion of new life
forms brought about by two events;

·first, the destruction of a large portion of
currently existing life to open up niches for new life forms to
occupy (as explained by the recent snowball earth), and

·second, the sudden development of an
environment very favorable to life produced an evolutionary pressure
toward new and different animals.

Notice that this hypothesis
never addresses the crucial of exactly how extra food might induce
development of a new animal phylum, but rather merely assumes that
our knowledge is too fragmentary concerning evolution to understand
fully the events involved.

Any explanation that tries to explain the Cambrian explosion
using methodological naturalism or evolutionary principles must also
explain why there have been no new phyla created since the
explosion. The Cambrian explosion is characterized by the
sudden appearance of vast quantities of hugely different and largely
unrelated life forms that apparently could not have arisen from each
other. Any explanation that tries to demonstrate how this
explosion occurred must also explain why, under apparently favorable
life sustaining conditions over the intervening millions of years,
no new animal phyla have arisen. The lack of a realistic
explanation becomes even more acute when history subsequent to the
Cambrian explosion is considered. During the intervening 543
million years, there have been several other major mass extinction
events when majorities of species living on the Earth were
destroyed. The most catastrophic of these was the
Permo-Triassic mass extinction of about 250 million years ago which
eliminated an estimated 90% of marine invertebrate species.Even after this mass
extinction, no new phyla appeared even though the number of species
approximated levels similar to the very low species diversity found
before the Cambrian explosion. The development events during
the Cambrian and Early Triassic period are dramatically
different.Both
produced new species, but the Cambrian event resulted in the
formation of many new body plans whereas the Triassic event resulted
only in the formation of several new species using body plans that
were already well established.

The Cambrian explosion is also characterized by the sudden
appearance of complete ecosystems where there are predator and prey
relationship, proper food for the various animals, and highly
complex interactions among animals. Thus, it would seem as
though the animals which did appear during this brief time interval
were those which could survive with each other. Indeed, an
ecosystem implies mutual benefit among the creatures involved so
that they helped each other survive and flourish in their particular
environment. The evolutionist then not only has to explain how
the Cambrian explosion happened whereby tens of new phyla suddenly
appeared (a phenomenon which has not since been duplicated), but
also how these brand new animals seemed to be made for each
other.The hypothesis
that seems to fit best with the data is that there was a Creator
which so fashioned these animals, for the Cambrian explosion is just
what one might expect under these circumstances.

Naturalists have used the recent discovery of multiple new
planets in nearby star systems to imply the ease with which life
might spring forth.Indeed, if Earth-like planets are plentiful then so must life
be plentiful throughout the galaxy and Universe. Most of the planets
discovered so far would be very poor candidates indeed for life as
they are very large planets with eccentric orbits around their
star. These planets would not be at all similar to Earth and
they would generate conditions that could not support carbon based
life. Indeed, it turns out that not only must the planet size
be similar to the Earth in order to sustain life, but it must be
similar to the earth in several other ways as well. Perhaps
the most surprising requirement for intelligent life is the
necessity for plate tectonics.

It is interesting when looking at mountains on other planets
in our solar system that such mountains are always single and never
form in chains. There is no equivalent to the Rockies, the
Andes, the Himalayas, or the other linear mountain ranges that are
so common on this planet. It seems odd to consider that
plate tectonics could be a key to the evolution and preservation of
life on Earth. The following factors will illustrate this
point,

·First, plate tectonics promotes high levels
of global biodiversity. The major defense against mass
extinction is high biodiversity, and the factor on Earth that is
most important toward maintaining biodiversity is plate
tectonics.

·Second, plate tectonics proves to be the
Earth’s global thermostat by recycling chemicals crucial to keeping
the volume of carbon dioxide in our atmosphere relatively uniform,
and thus it has been important in enabling liquid water to remain on
the Earth’s surface for more than 4 billion years,

·Third, plate tectonics is the dominant force
that causes changes in sea level, which are vital to the formation
of minerals which keep the level of global carbon dioxide (And
global temperature) to remain constant,

·Fourth, plate tectonics created the
continents upon which animal life might thrive. Without plate
tectonics, Earth might look much like it did after the first billion
years of its existence; a watery world with only a few isolated
volcanic mountains.

·Finally, plate tectonics makes possible some
of our planet’s most potent defense systems; the magnetic
field. Without the magnetic field, the Earth and its cargo of
fragile life, would be bombarded by a constant barrage of
potentially lethal cosmic rays, radiation, and solar wind.

Geologists of the last century had little difficulty
understanding the nature of volcanoes. Hot magma from deep
within the Earth rose to the surface regions and spewed forth lava,
ash, and pumice to form a cone. However, geologists had much
more difficulties understanding how non-volcanic mountains
formed. In 1910, an American geologist Frank B. Taylor
proposed the continents were drifting upon a soft cushion and when
these continents collided with each other, great mountains sprang up
at their junction. This theory was considered heresy and was
immediately decried by nearly all other geologists and geophysicists
who could envision no means whereby this drift might occur.

Taylor’s hypothesis, however, kindled a spark of interest
that would not be suppressed. Soon, other scientists began to
toy with the idea and searching for supporting evidence. The
most successful of these new converts to tectonics was Alfred
Wegener who, from 1912 until his death in 1930 was obsessed with
this idea. Wegener was the first to show how the fit of
various continents suggested an idea that all continents were once
united into a single super-continent. Furthermore, Wegener
used paleontological arguments to strengthen his claim. He
noted that presence of similar fossil species on land masses now
widely separated could have come about only if the various
continents had once been in contact. The greatest barrier to
the theory of plate tectonics was the lack of any mechanism whereby
continents might move. It was eventually discovered that the
answer to this question relies on the different phase states of the
Earth’s layers. These different layers are known as the crust
and upper mantle, and the presence of thermal convection to these
regions. Arthur Holmes proposed that the upper mantle might
act like boiling water and provide moving cells of material upon
which the continents could drift. The fluid rises as it is
heated, and when it eventually gets close to the surface it either
spews forth in a volcanic disruption or it begins to flow parallel
to the Earth’s surface. This material then cools and sinks
back down into the planet to be reheated once more.

Evidence has subsequently come forth from many sources that
the theory of continental drift was correct. We now understand
that all continents are masses of relatively low-density rock
embedded in a ground mass of more dense material. The low
density rocks have an average composition of granites whereas the
higher density rocks which make up the ocean crust are basaltic in
composition. Because granite is less dense than basalt, the
granite-rich continents essentially “float” upon a thin bed of
basalt. The Earth has a radioactive interior that constantly
generates great quantities of heat as the radioactive elements break
down into their various isotopic by-products. The heat which
is produced by this radioactive decay then rises toward the surface
and creates great convection cells of hot, liquid rock in the mantle
just as Arthur Holmes described. These gigantic convection
currents carry the thing brittle outer layer – known as plates –
upon them.

Many kilometers below the Earth’s surface, the familiar rocks
of our crust are exposed to great temperatures and pressures which
make them behave in different ways from that with which we are
familiar. The plates are of varying thickness, and their
“bottom” is thought by many scientists to coincide with a 1400
degrees Centigrade isotherm where the rock melts into a plastic-like
medium. The difference in viscosity between the overlying
plate and the underlying region of lowered viscosity allows the
relatively rigid crust to slip as a unit over the mantle.
Plates composed of oceanic crust and mantle are about 50 kilometers
thick, whereas the plates with continental crust average about 100
kilometers in thickness.

We will first discuss the ocean basins to understand the
process of tectonics. The crust lining the bottom of the
oceans is largely composed of basalt, the same type of volcanic rock
that makes up the Hawaiian Islands. This material originates
within the deeper mangle regions of the Earth where it is heated by
radioactivity and ascends toward the surface along rising zones of
the convection cells. As this dense mantle material rises
toward the surface, it moves into regions of ever decreasing
pressure and the lower density liquid separates from the higher
density material rising to the surface as “lava.” This lava
then enters a huge crack in the surface of the planet formed by the
pulling apart of two plates when it comes into contact with the
ocean water and solidifies into basaltic ocean crust. The
plates then move further apart and more lava then rises to take its
place forming a slowly moving conveyor belt of solidified basaltic
material.

The basalt that forms at the bottom of the oceans in the
cracks forming between separating plates has a lower density than
the basalt deeper in the mantle because it contains a much higher
percentage of silicon. This basalt has differentiated
from its parent compound called peridiotite. This
differentiation from a peridiotite composition to a basaltic
composition is the final step of oceanic crust formation. The
composition of continents is different from that of the ocean floors
being composed mainly of granite and andesite. Granite has a
characteristic speckled appearance compared to the brown to black
color of basalt. This speckling comes from their containing
even more of the white silica compound. The major step in the
formation of continental crust is the differentiation of granite
from material of a basaltic composition. This process takes
place in several steps, but the key ingredient is water, and the
process is that of subduction.

Over many millions of years, the oceanic crusts formed from
basalt rock gradually moves apart from its birthplace as newer
basalt rock gets laid down.Eventually, however, the spreading ocean flow can spread no
further. As the basalt rock cools it gets denser; also, it
acquires some heavy contaminants over time of dense igneous rock
called gabbro. Eventually, the additional weight of the
solidifying basalt rock and gabbro causes it to sink, down to as
deep as 650 kilometers. Eventually, the convection cell begins
its downward journey back into the deep mantle carrying its veneer
of oceanic crust back down with it in regions known as subduction
zones.

Subduction zones are long, linear regions where oceanic
crustal material is driven deep into the Earth, apparently not so
much as being pushed down but rather by sinking. It is near and
parallel to these subduction zones where mountain ranges
occur. These mountain ranges form partly as a byproduct of the
collision of two plates which causes the leading edges to crumple
and buckle, and partly because of the upwart movement of hot
magma. This magma eventually solidifies into granites and
other magmatic rocks parallel to the subduction zones. Most of the
world’s volcanoes are located near these subduction zones further
bearing testimony to the fundamental link between subduction and
mountain building. Mountain ranges are not found on any other
planet in the solar system illustrating that only the Earth has
tectonic plates and subduction zones.

Volcanoes occur along subduction zones due to the different
composition of magma and basalt.Basalt when first formed at
the ocean bottom is unhydrated. However, water gradually seeps
into the rock and is added to the crystalline structures of the rock
hydrating the basalt. Water poor minerals actually can
incorporate rather large amounts of water into their
structure. The newly hydrated minerals have a lower melting
point when compared to the unhydrated variety, so as the hydrated
basalt descends into the subduction zone, it is readily melted and
rises back toward the surface. The magma from a volcano
eventually cools into andesite (named after the Andes mountains),
and granite. The crucial fact is that the granite that makes
up the continents is of lower density due to its higher silicone
content than the basalt upon which it rests, and which is on the bottom of the oceans. The less dense continents float on a sea
of basalt and can never be sunk into a subduction zone.
Continents once formed cannot be destroyed (although they can be
eroded). The continents can be split and fragmented to drift
from one place to another, but their basic volume cannot be
significantly reduced.Through time, in fact, the number and volume of continents on
the Earth has gradually increased. Geologist Davie Howell, in
his book Principles of Terrane Analysis, estimates that eh
volume of continents increases by about 650 to 1300 cubic kilometers
of rock each year.

Tectonic plates can intersect with each other in three
different ways:

·The bottom of oceans where spreading two
plates spread apart; hot magma then arises between the two plates
producing new ocean floor,

·Areas where the plates grind side by side
such as at the San Andreas fault,

·Regions where plates collide at the
subduction zones, which are associated with linear chains of active
volcanoes such as the Cascades and the Aleutian
Islands <![endif]>

The majority
of Earth’s biodiversity today is based on continents and there is no
reason to believe that this relationship has changed over the past
several hundred million years. As the continents have grown
through time, they have affected the planet’s overall reflectivity
(albedo), he occurrence of glaciation events, oceanic circulation
patterns, and the amount of nutrients reaching the oceans. All
of these factors have biological consequences and affect global
biodiversity. Plate tectonics promotes biodiversity by
increasing the variety of habitats in which animals can life.
A world with mountainous continents, oceans, and myriad islands such
as those produced by plate tectonics produce a far more complex
world than would totally land or ocean dominated planets.
Furthermore, changes in continental position would affect ocean
currents, temperature, seasonal rainfall patterns, the distribution
of nutrients, and patterns of biological productivity.The deep sea is the area on
Earth that has the least biodiversity; over two-thirds of all animal
species life on land, and the majority of marine species live in the
shallow-water regions that would be more affected by plate tectonic
movements.

As
continental positions gradually change through time, the relative
abundance of north-south and east-west coastlines can change.Also, the larger the
continents the lower the environmental heterogeneity and the fewer
species and less biodiversity that would be formed. If many or
all of the continents were welded together into a super-continent,
biodiversity with would be expected to be lower than if the land
masses were broken up into many smaller areas.

One of the
most interesting facets about plate tectonics and why we have dwelt
upon this arcane topic is that it is probably essential for any form
of advanced life to exist on a planet. For animal life based
upon DNA to exist, water is necessary and must be abundant on a
planet’s surface. In desert environments, there is little
biodiversity, but in rainforests in the same latitude, biodiversity
is considerable. For complex life to be attained and then
maintained, a planet’s water supply must,

·Be large enough to sustain a sizable ocean
on the planet's floor,

·Have migrated to the surface from the
planet’s interior,

·Not be lost to space, and

·Exist largely in liquid form.

Plate tectonics play a role in all four of these
criteria.

Earth is about 0.5% water by weight, and must of this water
arrived among the planetesimals that took part in the planet’s early
development and accretion. Incoming comets dumped other
volumes of water here after the Earth accreted. The relative
importance of these two major processes is largely unknown at this
time. Once water has arrived on the surface of a planet, its
maintenance becomes the primary requirement for attaining and
sustaining animal life. The maintenance of liquid water is
controlled largely by planetary temperatures, which are by-product
of the greenhouse gas volumes of a planet’s atmosphere.The temperature of Earth
(and other planets) is determined by several factors. The
first is related to the energy input form the sun. The second
is a function of how much that that energy input from the sun is
reflected back into space. The third is related to the volume
of “greenhouse gases” maintained in a planet’s atmosphere. The
volume of greenhouse gases in the atmosphere is not constant but is
are eventually broken down or undergo a change in phase; therefore,
if supplies are not constantly replenished, the planet will grow
colder until the freezing point of water is reached. Once
water freezes, then the planet will grow colder rapidly as ice will
reflect heat back into space more efficiency than does liquid
water. The most important sources of planetary greenhouse
gases are volcanic eruptions which occur on planets with or without
tectonic plate systems. Even so called “dormant” volcanoes are
venting carbon dioxide into the atmosphere in great
quantities. On any planet with volcanism there is usually an
abundance of greenhouse gases – too much in some cases (perhaps
becoming so in the case of the Earth!), and this is where plate
tectonics becomes crucial.

Greenhouse gas composition are byproducts of complex
interactions among the planet’s interior, surface, and atmospheric
chemistry. One of the most important by-products of plate
tectonics is the recycling of mineral and chemical compounds bound
up in any planet’s sedimentary rock layer. On non-plate
tectonic worlds, vast quantities of sedimentary material are formed
by erosion and then form layer upon layer of rock, permanently taken
out of planetary circulation. The only means for these
sedimentary rocks to be exhumed is through some process leading to
mountain building. However, mountain building on non-plate
tectonic worlds is limited to the formation of large volcanoes over
hot spots. However, with plate tectonics, the interaction of plates,
formation of mountain chains, and the process of subduction all lead
to a recycling of many materials. One of the most important of
these materials is putting carbon dioxide back into the
atmosphere. As limestone is subducted deep into the mantle, it
changes in the process returning large amounts of carbon dioxide
into the atmosphere. Since carbon dioxide is a greenhouse gas,
this leads to global warming keeping liquid water in the liquid
state.

The burning of large amounts of hydrocarbon fuels has largely
produced the recent onset of global warming on our planet. We
have become more interested in the reverse process; that is global
cooling. The most important element in reducing atmospheric
carbon dioxide is the weathering of minerals known as silicates such
as mica and feldspar (contained within granite). The basic
chemical reaction is CaSiO3 + CO2 = CaCO3 + SiO2. When the
first two chemicals in this equation combine, limestone (calcium
carbonate) is produced and carbon dioxide removed from the
atmosphere. The basic process responsible for getting granite
to undergo this chemical reaction is weathering. The rate of
chemical weathering increases as a planet warms as a warmer planet
produces more rain and severe weather that in turn produces more
weathering of granite surfaces. As the rate of weathering
increases more silicate material is made available for reaction with
the atmosphere and more carbon dioxide is removed thus causing
cooling.Similarly, as
the planet cools, weathering will reduce and the carbon dioxide
content of the atmosphere will begin to rise causing warming to
occur. The carbon-silicate weathering cycle cannot work
efficiently on planets without plate tectonics.

Cosmic rays are high-energy particles that constantly bombard
the earth, traveling at velocities approaching the speed of
light. They come from many sources, including the sun and
sources from deep space including supernovae, the explosions of
stars. These catastrophic events send great numbers of
high-energy particles into space some of which bombard the
Earth.

In The Search for Life in the Universe, by D.
Goldsmith and T. Owen, the authors speculate that without some form
of protection from these high-energy particles, life on earth would
be extinguished within several generations. Fortunately, the
vast majority of these particles are deflected by the Earth’s
magnetic field. The innermost layer of our planet – the core –
is made up of mainly iron in the liquid state. As the planet
spins, it creates convective movements in this liquid that produces
a giant magnetic field surrounding the entire planet. The
convections cells within the liquid core is enabled by heat loss;
heat must be exported out of the core in order for the whole system
to work. Joseph Kirschvink of Cal Tech has suggested that
without plate tectonics, there would not be sufficient heat loss
across the core region to produce the convective cells necessary to
generate Earth’s magnetic field; thus, there would be no convective
currents and no magnetic field. Furthermore, the magnetic
field around the earth protects our atmosphere from being blown away
by solar wind – particles periodically aimed at the earth from the
sun in great volumes produced by solar storms. There is good
reason to believe that without plate tectonics, there would be no
magnetic field and perhaps no animal life.

Plate tectonics plays at least three crucial roles in
maintaining animal life; it promotes biological productivity; it
promotes diversity; and it helps maintain equable temperatures
necessary for animal life. It is difficult to determine at
this time the rarity of plate tectonics. However, there is
evidence that the emergence of plate tectonics on Earth depended
upon the presence of a large companion moon.

The
adequate abundance of silicates for carbon dioxide removal requires
an active plate tectonics system. The Earth needs three things
for plate tectonics to occur: a stable dynamo
(electromagnetic generator) at its core, a powerful interior source
of radioactive decay, and an abundant supply of liquid surface
water. The presence of any one of these processes would be
"unexpected" by natural processes but for the three of them to be
present simultaneously is mind boggling.

The
Earth's dynamo works with enduring efficiency
because several independent factors fall within certain narrow
ranges. These factors include,

·Solar and
lunar gravitational torques,

·The frequency
or period of the core’s gyrations (its “precession”),

·The ratio of
the inner core radius to the outer core radius,

·The relative
abundances of silicon, iron, and sulfur in the solid inner
core,

·The ratio of
inner core magnetic diffusivity (a measure of how well a magnetic
field diffuses throughout a conducting medium) to outer core
magnetic diffusivity,

·And the
viscosity of the material at the boundaries between the solid inner
core and the liquid outer core, also between the liquid outer core
and the mantle

The
Earth's radioactive elements are also necessary to provide the
long-term heat needed for volcanism to occur - also important in
plate tectonics. Two very unlikely events brought the
necessary radioactive elements to the earth for this to occur.
First, the gas cloud that condensed into the Sun and its planets
formed adjacent to both the fresh remnant of a Type 1
supernova and the fresh remnant of a Type II supernova. Each
contributed radioactive and life essential heavy elements to the
emerging solar system. Second, between 4.4 and 4.5 billion
years ago, a Mars
sized object crashed into the Earth hitting at the proper speed,
angle, and location to transfer its radioactive and other heavy
elements to the Earth's interior. The lighter material of both
the collider and the Earth formed a debris cloud that eventually
condensed into the Moon.

A
problem arises, however, because radioactivity naturally declines
with time. Therefore, the energy released from radioactive
decay contributed less and less toward maintenance of plate tectonic
activity. Thus, if erosion were to continue at the same rate,
eventually all the continents would be eroded into the ocean.
However, the Moon acts as a tidal brake with its gravitational tug
gradually slowing the Earth's rotation rate. Strategically,
this reduced rotation rate results in a just-right reduction of
erosion.

Life
itself has also provided a critical piece in the Earth temperature
stabilization picture. The essential species and the entire
matrix of life forms supporting their existence - in other words,
entire ecosystems - existed at the right population levels in the
right locations at the right times to assist in controlling the
quantity of greenhouse gases, that in turn has kept the Earth's
temperature in life's safe range for nearly four billion
years. Furthermore, this regulation of the Earth's surface
temperature in the context of a steadily brightening Sun mandates a
carefully timed progression of new life forms. For example,
the most advanced plants on Earth have vascular bundles which are
more efficient than other more primitive plant species in
accelerating erosion. Thus, as plate tectonics and erosion
gradually decline, Earth needs more and more of these advanced
plants to sustain adequate carbon dioxide levels to maintain proper
temperature. This increase in advanced plant forms has
coincided with a commensurate reduction in more primitive forms
which are less efficient in producing erosion.

Perhaps one of the most wonderful and unique
relationships in natural science has been played out over the past 4
billion years since our current solar system came into being.
This relationship is between the Earth and the Sun, and concerns how
the Sun has been gradually brightening over that time period, and
how the Earth has been simultaneously and independently changing so
as to assume a relatively stable temperature to support the
emergence of life. Without this delicate balance, the Earth
would have become either too hot or too cold to allow for the
emergence of intelligent life. While we worry today about
global warming and the influence this warming might have upon the
environment, a much greater threat to life on Earth has existed over
time with the gradual brightening of the Sun.

The
Sun was about 30% less luminous about 3.8 billion years ago when
life first emerged on this planet than it is now. Knowing that
a drop of only 1-2% of the Sun's brightness would plunge the planet
into an overwhelming ice-age (assuming current atmospheric
conditions), or an increase of only 1-2% would boil away the oceans
and cook all life, the question emerges as to how life survived
during an increase in the Sun's luminosity of 30%. This has
been called the "faint Sun paradox" and emerges as one of the most
elaborate and wonderful interplay of forces to permit a stable Earth
environment.

The
Sun was born about 4.5 billion years ago with the gravitational
collapse of a huge gas cloud, and with this collapse became
progressively more hot until nuclear fusion could occur.
Initially, these nuclear reactions were unstable causing the
luminosity of the Sun to fluctuate widely. Furthermore, during
the initial 500 million years of the Sun's existence, the emission
of ionizing radiation - in particular X-rays - was fifty times
greater than current levels. Thus, during the first
half-billion years of the Sun's existence, the Earth would have been
a very inhospitable place for the emergence of life.

Temperature at the core of the Sun reached 17 million
degrees Centigrade igniting the fusion of hydrogen into helium
gradually increasing the amount of helium. A greater
percentage of helium causes the nuclear furnace to burn more
efficiently gradually increasing core temperature. With
greater efficiency of nuclear fusion the Sun's surface temperature
also gradually increased as well and will further increase until all
of the hydrogen at the Sun's core has been converted into
helium. This whole process takes about 9 billion years for a
star the size of the Sun, so we are now at about exactly the
half-way point.

The
paradox that exists, however, is even though the Sun's luminosity
was about 30% less than current levels 3.8 billion years ago when
life first emerged, the surface temperature of the Earth was only
marginally different than today. Both liquid water and life
began to abound at this point, both of which would be impossible
were the Earth's surface temperature different. This balancing
act continued for the next nearly 4 billion years whereby the Earth
compensated for different solar luminosity through a complicated
series of environmental changes.

Initially, the reduced luminosity of the Sun was
compensated for by an increase in greenhouse gases, especially
carbon dioxide. These gases served to insulate the earth
conserving heat from the less luminous sun producing a climate which
approximates our own. As the ancient Sun slowly brightened,
there was a continuous supply of silicates exposed to the atmosphere
(containing silicon, oxygen, and metals that comprise more than 90%
of the Earth's crust), and a continuous burial of carbon-rich
organic matter the decomposition of which would produce additional
carbon dioxide.

In
the presence of liquid water, silicates chemically react with carbon
dioxide from the atmosphere forming carbonates and sand in the
process. As exposed silicates react with carbon dioxide, new
silicate deposits need to be brought up to the surface. This
requires the plate
tectonic system to push silicates above the ocean floor
producing islands and continental land masses. Then, erosion
must "plough" the crust so that more silicates are constantly
brought into contact with the atmosphere. Erosion is a
complicated process. Multiple processes determine its
efficiency including the Earth's rotation rate, average rainfall,
average temperature, average slope of the land masses, and the types
and quantities of plant species on the land masses. If erosion
proceeds too slowly, silicates cannot maintain an adequate pace of
carbon dioxide removal - too much erosion removes too much, too
quickly.

Organisms also are involved in the planetary heat
stabilization process. Photocynthetic plants, plus bacteria
and methanogens (which remove methane from the atmosphere - another
greenhouse gas), also work to take water, methane and carbon dioxide
from the atmosphere chemically transforming them into fats, sugars,
starches, proteins, and carbonates. If these compounds get
buried before they can decay or be eaten by other organisms, they
help in the task of reducing greenhouse gases. (Humans have
benefited greatly as they form a wealth of biodeposits such as
limestone, marble, fossil fuels, and concentrated metal
ores).

There is
strong likelihood that without the Moon and Jupiter, animal life
would not have developed on Earth. Both are key elements for
different reasons.

The Moon. The
likelihood that an Earth sized planet would have such a large moon
is small as the conditions suitable for moon formation were common
for the outer planets but rare for the inner ones. Of the many
moons in the solar system, nearly all orbit the giant planets in the
outer solar system. The only moons of the terrestrial inner planets
are Phobos and Diemos – two tine (10 kilometers in diameter) moons
of Mars. Some of the moons orbiting the larger planets are
huge; Ganymede orbiting Jupiter is nearly as large as Mars, and
Saturn’s Titan is nearly that large but also has an atmosphere more
dense than our own (although much colder). The Moon is nearly
a third the size of the Earth, and in many respect is more like a
twin than a moon. The only other case in the solar system with
a moon comparable in size to its planet is in the case of Pluto and
its moon Charon.

The Moon
plays three roles that affect the development and survival of life
on Earth,

·It controls tides,

·It stabilizes the tile of Earth’s spin
axis,

·It slows the Earth’s rate of rotation

Of
these three factors, the most important is its effect on the tilt of
the Earth’s spin axis relative to the plane of its orbit that is
called its “obliquity.” Obliquity is the cause of seasonal
changes, and for most of the Earth’s history, its obliquity has not
varied by more than a degree or two from its present value of 23
degrees.Although the
direction of the tilt varies over periods of tens of thousands of
years as the planet wobbles – much like a spinning top – the angle
of the tilt relative to the orbit plane remains almost fixed.
This angle is nearly constant for hundreds of millions of years
because of the gravitational pull of the Moon. Without the
Moon, the tilt angle would wander in response ot the gravitational
pulls of the sun and Jupiter. The montly motion of our large
Moon damps any tendencies for the tilt axis to change. If the
Moon were smaller or more distant or if Jupiter were larger or
closer, or if the Earth were closer to or father away from the Sun,
the Moon’s stabilizing influence would be less effective.
Without the stabilizing effect of the Moon, the Earth’s spin axis
might vary by as much as 90 degrees. Mars, a planet with the
same spin rate and axis tilt but without a large moon, is believed
to have exhibited changes to its tilt axis of 45 degrees or
more.

The tilt of a planets spin axis determines the relative
amount of sunlight that fall on the polar and equatorial regions
during the seasons and strongly affects a planet’s climate. On
planets with a moderate spin axis, the majority of solar energy is
absorbed in the equatorial regions where the moon is always high in
the sky. The poles of such a planet are in total darkness for
half a year and constantly illuminated for half a year. The
highest altitude that the sun reaches in the sky at the pole is
exactly equal to the number of degrees tilt of the spin axis.
The planet Mercury provides an example of what can happen on a
planet whose spin axis is nearly perfectly perpendicular to the
plane of its orbit. Mercury is the closest planet to the sun
and most of its surface is very hot, although the poles are covered
with ice. The planet is very close to the sun, but as viewed
from the poles, the sun is always on the horizon. In contrast
to Mercury’s lack of tilt, the planet Uranus has a 90-degree tilt,
and one pole is exposed to sunlight for half a year while the other
experiences extreme cold and darkness.

Constancy of the tile angle is the factor that provides
long-term stability of the Earth’s surface temperature. If the
polar tilt axis had undergone wide deviations from its present
values, Earth’s climate would have been much less hospitable for the
development of higher life forms.One of the worst
possibilities is that the excessive tilt axis could have led to the
total freezing over of the oceans which might be difficult to
recover from. Astronomer Jacques Laskar noted,

“These results show that the situation of the Earth is
veruy peculiar. The common status for all the terrestrial
planets is to have experienced very large scale chaotic behavior for
their obliquity, which in the case of the Earth and in the absence
of the Moon, may have prevented the appearance of evoluted forms of
life. … We owe our present climate stability to an exceptional
event: the presence of the Moon.”

In the
distant future, the Moon will lose its controlling influence upon
the Earth’s spin axis. The Moon is slowly moving outward from
the Earth at the rate of about 4 centimeters a year), and within 2
billion years it will be too far away to have enough influence to
stabilize Earth’s obliquity. The Earth’s tilt angle will begin
to change as a result, and the planet’s climate will follow.
Furthermore, the sun’s increase in brightness will continue over the
years, so that at the time when our planet’s spin axis begins to
wander, the sun will be hotter, and both effects will decrease the
habitability of the Earth.

Lunar
Tides. The Earth also experiences tides which are due to
the gravitational effects of both the sun and the Moon. These
two bodies produce bulges in the ocean pointing both toward and away
form the Moon and the sun. The complexity of the Earth’s
present tidal effects is well known by those making their living
upon the ocean. The effect is complicated as both the Moon and
the sun cause ocean pulges on their respective near and far sides of
the planet. When the Moon lines up with the sun every two
weeks, the tidal changes are at a maximum, and when they are 90
degrees apart in the sky, the range of tidal change is
minimized. With a smaller or more distant moon, the lunar
tides would be lesser and would have a different annual
variation.

Soon after the Moon formed, it was perhaps 15,000 miles from
the earth. Instead of tides being a few meters high like they
are today, they might have been hundreds of meters high with the
moon was first created. The extreme effects of such a close
moon could have strongly heated the Earth’s surface. There
would have been tremendous flexing and torsion of the Earth’s crust,
along with frictional heating which may have actually melted the
rocky surface. The retreat of the Moon is a natural
consequence of gravitational pull between the Moon and the tidal
bulges. The Earth’s lunar tidal bulges don’t actually
correspond with a line drawn from the Earth to the Moon, but rather
lead ahead as the Moon orbits the planet. This offset produces
a torque that causes the Earth’s spin rate to decrease slowly and
the distance between the Earth and the Moon to slowly
increase. As we have previously noted, laser measurements
indicate that the Moon is receding from the earth at the rate of
about 4 cm/year. As the Moon gets further away, the Earth
spins slower; coral records show that about 400 million years ago
there were about 400 days in a year; the Moon was closer to the
Earth and the Earth was spinning faster. The coupling of these
two effects is due to conservation of angular momentum, the same
physical property that allows ice skaters to spin faster by pulling
their arms closer to their bodies. Interestingly, Triton, the
large moon of Neptune, is in a retrograde orbit causing it to
gradually move closer to its planet. It will collide with
Neptune in a few hundred million years.

The Moon’s Origin. We have seen
how the Moon stabilizes the Earth’s rotational axis and the seasons
unlike the other planets. And, we have also seen that the Moon
itself is somewhat unusual with respect to its size; it is very
large relative to the planet about which it rotates. This
leads us to the possibility that the origin of the Moon was also
unusual – and this indeed appears to be the case.

Before the Apollo Moon rocks were brought back home, the most
popular theories regarding origin of the Moon held that it had
formed cold and consequently that the rocks would retain records of
the earliest history of the solar system. Theories on the lunar
origin usually fit into three categories; it formed in place, it
formed elsewhere and then was captured by the Earth, or it was
somehow ejected from the Earth.The newest model that seems
to best fir the data indicates that a Mars-sized projectile hit the
Earth. Debris from this collision was ejected into space, and
some if it remained in orbit where self-collisions would cause it to
form a thin, orbiting ring of rocks similar to the rings of
Saturn. The Moon would form by collision and sticking – the
processes of accretion that built most of the bodies in the solar
system.

There are several observations which are consistent with this
collision model of lunar formation. First, the moon is
depleted of elements such as zinc, cadmium, and tin. These are
relatively volatile elements which would have been vaporized in the
impact, and the resulting vapor would not have condense well in the
heat of the moon’s surface and would then have been swept away into
space and lost to the Earth-moon system. Other elements and
compounds that would have been lost in this collision include
nitrogen, carbon, and water. One of the surprising findings
from the Apollo mission was the lunar samples were exceedingly
dry. Unlike rocks from the Earth, they contained no detectable
water – not even in crystalline form.Another remarkable property
of the lunar samples was they are highly depleted in the
iron-binding elements that tend to concentrate in the metallic iron
cores of planets. When planetary cores form from molten iron
sinking into the planet’s center, the iron-binding elements such as
platinum, gold, and iridium are incorporated into the falling iron
and are highly depleted from the crustal and mantle rocks.
That the lunar rocks was similarly depleted of these iron-binding
elements was unexpected because the moon does not have a significant
iron core. The mean density of the Moon is 3.4 times water,
very similar to that of rocks on the lunar surface, and much lower
than that of the Earth which is 5.5 times water. If the moon
had a significant iron core, then its mean density would be much
higher than that observed. Additionally, seismic and magnetic
data also indicate no evidence of a significant iron core.
Finally, the moon and Earth are very similar with respect to
similarities in trace element concentration as well as the
composition of various isotopes.

The Moon apparently formed as the result of a collision of a
that was about a fourth of the final mass of the Earth, with an
Earth that was about half its current size. The collision
between these two masses briefly resulted in a single mass; however,
inertial effects literally ripped the mass apart resulting in
material being shed into space. Some of this material then
fell back to Earth, while others accreted to form the Moon over a
few tens of thousands of years. When the Moon did form, it was
about 15,000 miles away from the Earth’s surface. With the
Moon so close, the Earth would have been spinning at a rate that a
day would only be five hours long. The height of the tides
would have been fantastic – hundreds of feet – resulting in an
inertial drag upon the Earth that would rapidly have slowed its
rotation. It is possible that the Earth’s violent history was
responsible for its plate tectonic structure.

Jupiter is a giant ball of gas consisting of mostly hydrogen
and helium. In the interior of the planet, the gas acts more
like a metal as the electrons are not restricted to one atom but
rather move freely from atom to atom.At pressures of a million
atmospheres in the center of Jupiter, hydrogen is in a metallic
state.

Jupiter is over 300 times the mass of the hearty and is by
far the most massive of the planets orbiting the sun. Jupiter
formed from accretion of gas from the solar nebula in addition to a
few solids. Jupiter formed very quickly, and its rapid growth
had major effects upon other planets, especially those which were
trying to accumulate mass just inside its orbit. A terrestrial
planet was in the process of forming midway between Mars and
Jupiter, but because Jupiter formed first, its development was
aborted. This failed
planet is now the asteroid belt, a region where original
planetesimals and fragments still survive but were never assembled
into a planet. The largest of these rocks is the asteroid
Ceres – a rounded object 1000 kilometers in diameter.

The asteroids are the source of meteorites, and detailed
examinations of these ancient rocks gives insight into the nature of
planet formation. These rocks from heaven are the oldest
radiometrically dated rocks, and suggest that while initially there
was a period of growth when colliding materials led to accretion but
that later, during most of the solar system’s history, collision of
rocks have occurred with such high energy that they led to erosion
and disruption – not growth. It has been asserted that without
the influence of the giant planet Jupiter, both Mars and the
asteroid planet would have grown to the size of the Earth.

Mars might have developed into a planet very similar to the
Earth; however, because of the gravitational effects of Jupiter,
fewer mass accretions occurred retarding Mars’ growth. Because
Mars is a much smaller planet, it did not develop a significant
metallic core; therefore, it has only a small magnetic field.
The insignificant magnetic field around Mars could not protect the
planet’s atmosphere from depletion due to solar wind. The
depleted atmosphere was then unable to protect Mars from solar and
cosmic radiation which over time would sterilize the planet’s
surface making it ever more difficult for life to appear.
Similarly, if the Earth had been a little closer to Jupiter, or if
Jupiter had a somewhat larger mass, then the “Jupiter effect” that
aborted the formation of the asteroid planet and reduced the size of
Mars would also have affected Earth rendering it a smaller
planet. If the Earth had been smaller, then it too might not
have been able to keep its atmosphere and oceans rendering its
long-term suitability for life much less ideal than present
conditions.

Jupiter also played a role in reducing the number of large
asteroids in the inner solar system, objects which might have
produced sterilizing impacts with the Earth. Jupiter is 318
times more massive than the Earth, and its enormous gravitational
influence even impacts the inner reaches of the solar system.
The early history of the solar system produced enormous numbers of
asteroids which failed to coalesce into a planet and which might
impact the Earth with adverse results. However, over billions
of years, most of these asteroids have fallen victim to the
gravitational effect of Jupiter falling into the planet never to be
seen again.Alternatively, some of these asteroids were ejected out of
the solar system, into the Sun, or into the Oort cloud of
comets. Jupiter was the major cause of this purging of the
middle region of the solar system. The objects that still
survive today to impact the Earth are survivors of the gravitational
pull of Jupiter, and survive in three special ecological
regions:

·The Oort cloud beyond Pluto,

·The Kuiper belt of comets just beyond the
outer planets,

·The asteroid belt largely confined between
the orbits of Jupiter and Mars.

The
impact rate into the Earth of a 10-kilometer object occurs every 100
million years – the last one occurring about 65 million years ago
which is thought to have killed the dinosaurs and the time of the
K/T extinction. George Wetherill of the Carnegie Institute of
Washington has estimated that the rate of impact of these 10
kilometer objects might be 10,000 times as great if Jupiter had not
come into being and purged many of the leftover bodies out of our
region of the solar system. If the Earth had been subject to
impacts with 10-kilometer asteroids every 10,000 years rather than
every 100 million years, it seems very unlikely that life – let
along animal life – could have survived.

Special Formation of Jupiter. It is unclear
whether planets similar to Jupiter and Saturn form in solar systems
with planets similar to the Earth. Jupiter is unique in
several respects, however. First, Jupiter grew very quickly –
it grew to a mass of 15 Earths before Mars grew to 10% of an Earth
mass. The reason for this very rapid early growth is because
Jupiter formed in a region of the solar system where water exists in
the form of ice, whereas Earth and the inner planets formed in a
region of the solar system where water exists in the form of liquid
water. As ice, water helps the accretion process accelerating
the conglomeration of rocks into a larger body. Jupiter’s
growth to a giant planet first started with a giant core of rock,
metal, and ice, forming a 15 Earth mass body. At this size,
the gravity of the core could attack and hold hydrogen and helium,
the gases which account for 99% of the mass of Jupiter.

Not only are large planets like Jupiter and Saturn beneficial
for life on Earth as their gravitational pull clears asteroids from
the Earth’s vicinity, but also we are fortunate that the orbits of
these two planets is roughly circular. An irregular, very
elliptical orbit might bring these planets dangerously close to the
inner planets destabilizing their orbits with disastrous
consequences for any life that might be on them. Indeed, an
elliptical orbit for large planets may be the norm rather than the
exception. Many of the large planets that are being discovered
rotating about other stars have very irregular orbits.

We have so
far examined several factors (among others) which make the Earth an
ideal habitat for life

·its temperature,

·atmosphere,

·tectonics,

·percentage of land,

·position from the sun,

·sun that is not too hot, nor too large, does
not burn too quickly, but is increasing gradually in brightness over
time,

·earth axis tilt stabilization by the Moon,
and

·protection by Jupiter are some of these
positive influences.

What are the
odds that these same occurrences happened elsewhere in the Universe
to allow intelligent life as on Earth (at least we suppose
intelligence!)? In other words, how rare is the Earth?

In order to
try to get an estimation of the odds relating to the creation of a
planet like the Earth, it is necessary to create a solar system
similar to ours. The first major object that comes into
existence in a solar system is its sun; therefore, we start with the
odds relating to the creation of a sun smilar to ours.

The
Sun. We should probably start with similar ingredients
to that which formed our solar system; that is, the same mass and
elemental composition. According to most theorists, this might
create a star identical to ours – but then again, it might
not. The spin rate of the new star might be different from
that of our sun with unknown consequences. We need to develop
a star that has similar mass than ours; one that will burn long
enough to let live development occur, one that does not pulse or
rapidly change its energy output, one that does not produce too much
ultraviolet radiation, and one that is large enough – but not too
large. Of 100 stars, perhaps only two to five will yield a
star as large as our sun; the vast majority of stars in the Universe
are smaller than our sun. While smaller stars could have
planets with life, most would be so dim that Earth-like planets
would have to orbit very close in order to receive sufficient energy
to melt water. However, being close enough to get adequate
energy from a small star would lead to tidal lock whereby the same
side of the planet always faces the sun. A totally locked
planet is probably unsuitable for advanced life.

Planetary System.If we were to examine
planetary systems about a star that is similar to our sun, there
little chance that we would find one similar to ours. Namely,
it would be unlikely to locate a system with a Jupiter-like planet
and three other gas giants orbiting outside four terrestrial planets
with a halo of comets surrounding the entire mix. If we were
to examine 1000 planetary systems, there would likely be none that
would be identical to our solar system today. James Kasting
from Penn State University believes that planetary spacing is not
accidental but regulated by physics, and that if the solar system
were to reform many times, we would get the same number of planets
each time.But, the
extra-solar planets that have been discovered seem to exhibit an
enormous diversity of spacing and orbits with their positions not
being nearly so orderly as Kasting’s theory might suggest.
Ross Taylor states, “Clearly, the conditions that existed to make
our system of planets are not easily reproduced. Although the
processes of forming planets around stars are probably broadly
similar, the devil is in the details.” Indeed, no one knows
whether a planet the size of Jupiter would always form or whether
there would be a couple of planets like Mars. A planet would
probably form in a position similar to that of the Earth, but nobody
knows whether that planet might be smaller, larger, or in a slightly
different orbit or orbital eccentricity.

Even if all these events occurred more or less the way
they have with our solar system, would life form? And given
life, would animal life appear once more? Can there be animal
life without the utterly chance events that occurred in Earth’s
history such as a Snowball Earth, or an inertial interchange event,
for instance.

Thus, in summary, the set of questions are as
follows:

·How many of all planets in the Universe are
terrestrial planets (as opposed to giant gas planets such as Jupiter
and Saturn),

·What is the percentage for all planets of
the Universe?

·Of the terrestrial planets in the Universe,
how many of them have sufficient water to form an ocean,

·Of all the planets with an ocean, how many
have any land,

·Of those with land, how many have continents
(rather than scattered islands),

Similar questions abound for
each step of the developmental process.

The Odds of
Life

Frank Drake developed a thought-provoking formula which
sought to predict how many civilizations might exist in our
galaxy. The point of this formula was to estimate the
likelihood of detecting radio waves sent out from other
technologically advanced civilizations. How called the Drake
equation, it has had enormous influence to development of the Search
for Extraterrestrial Intelligence (SETI) project. The equation
is a string of independent factors that when multiplied together
give an estimate of the number of intelligent civilizations in our
galaxies.

·fl = percentage of a lifetime of a planet
that is marked by the presence of a communicative civilization.

When Drake first published his famous equation, there
were great uncertainties in most of the factors. The number of
stars in our galaxy is between 200 and 300 million. The number
of star systems with planets, however, was very poorly known in
Drake’s time (about 1950). Although many astronomers believed
that planets were common, that was only theory. Carl Sagan
estimated that an average of ten planets would be found around
each star.Now
that numerous stars have been examined, it appears that only about
5% of stars have detectable planets. Because in general, only
large gas-giant planets are detectable, this figure really shows
that Jupiter clones close to stars or in elliptical orbits are
rare. Spectroscopic examination indicates that stars that
appear to have planets also appear to be rich in metals similar to
our own sun. Thus, according to some astronomers conducting
these studies, there seems to be a causal link between high-metal
content in a star and the presence of planets. Our own star is
metal-rich; in a study of 174 stars, astronomer G. Gonzales
discovered that the sun was among the highest in metal
content. It appears as though we orbit a rare star after
all!

Other studies indicate that planetary systems such as our
own might also be rare. At a large meeting of astronomers in
Texas in 1999, it was announced that 17 nearby stars had been
observed to have planets the size of Jupiter orbiting them.
What seemed obvious, however, was that none of the planetary systems
being discovered seemed to be like our own. Most of the
Jupiter-like objects which orbit these stars travel in elliptical
orbits, not circular orbits – which is the rule in our solar
system. In such planetary systems, the possibility of an
Earth-like planet existing in a stable orbit is low indeed. A
Jupiter coming in close to the sun would have destroyed the inner
rocky planets by causing the inward planets either to spiral into
their sun or to be ejected out of their solar system into the cold,
dark space between stars.

Furthermore, it seems as though the frequency of planets
orbiting stars is probably much lower than originally
anticipated. As previously noted, Carl Sagan in 1974 estimated
that the average number of planets orbiting each star is ten.
This was echoed by Goldsmith and Owen in their 1992 book, The
Search for Life in the Universe.

The most common stars in the Universe are M stars –
fainted than the sun and nearly 100 times more numerous than
solar-mass stars. These stars can generally be ruled out as
having life near them because their “habitable zones” where surface
temperatures could be conducive to life are uninhabitable for other
reasons. To be warmed by these cool stars, the planets would
have to be so close to the star that tidal effects from the star
would force them into synchronous orbits with one side of the planet
always facing the star while the other side would be permanently
dark. On the other hand, stars which are much more massive
than the Sun would have stable lifetimes of only a few billion years
which would probably be too short for the development of advanced
life. Each planetary system around a star similar to our own
sun might not have an Earth-sized terrestrial planet orbiting the
star in its habitable space.When we take into consideration factors such as the abundance
of planets and the location and lifetime of the habitable zone, the
Drake Equation suggests that only between 1% and 0.001% of all stars
might have planets with habitats similar to those on the
Earth. Furthermore, of those stars similar to the sun with
planets similar to the Earth, how many of them have an Earth with a
large Moon and a Jupiter-sized planet in just the right position and
circular orbit to protect that planet.

We can now arrive at an estimation of the odds of finding
intelligent life by examining these factors,

·Stars in the Milky Way,

·Fraction of stars with planets,

·Fraction of metal-rich planets,

·Planets in a star’s habitable zone

·Stars in a galactic habitable zone,

·Fraction of habitable planets where life
does form,

·Fraction of planets with life where complex
animal life arises,

·Percentage of a lifetime of a planet that is
marked by the presence of complex animal life

·Fraction of planets with a large moon

·Fraction of solar systems with Jupiter-sized
planets,

·Fraction of planets with a critically low
number of mass extinction events

With these added elements, the
number of planets with animal life gets smaller and smaller.
Clearly, many of these terms are known in only the roughest details;
but it does seem that the Earth may indeed be extraordinarily
rare.

Sadly, life on Earth is coming to an end. The Sun
is getting progressively hotter and the greenhouse gases need to be
continuously reduced in order to reduce surface temperature.
However, there is a limit under which carbon dioxide - the most
important greenhouse gas - cannot fall as photosynthesis must
continue or all green plant life would die. The current carbon
dioxide level is 375 parts per million in Earth's atmosphere; when
the atmospheric carbon dioxide level falls below about 225 parts per
million, all photosynthetic life will die - followed shortly by all
animal life. Alternatively, if we continue with our polluting
ways dumping carbon dioxide into the atmosphere, the increased
greenhouse gases combined with a brightening Sun will kill all
animal life long before we have to worry about loss of
photosynthesis.

The timing of man's arrival on Earth toward the end of
its life sustaining capacity seems tragic at first. However,
upon closer inspection, we seem to have been provided a great
gift. Scanning the Earth's surface, we see great evidences of
man's presence; farms, ranches, towns, cities, transportation and
communication facilities, a plethora of building materials which are
all derived from nearly four billion years of life - and
death. From dead life we get gems, sand, steel, asphalt,
concrete, copper, limestone, marble, plastics, etc. Most of
the energy that drives civilization comes from the biosphere in the
form of petrochemicals, wood, kerogen, and so forth. Must of
the fertilizers that support agricultural production also come from
bio-deposits - phosphates, nitrates, and other chemicals.

Such a bountiful provision speaks of a great Provider who
carefully planned and prepared the planet through the ages for human
life. Furthermore, this Provider has given man the ability to
use the bio-deposits around him for his own benefit. But
perhaps even more important, we have also been able to use this
intelligence to understand the "big picture" - how the cosmos seems
to have been put together with fine detail and "just right" physical
laws to ensure man's survival. The more we understand about
the complexities of life, the fine tunedness of physical laws to
ensure this life, how this planet is engineered to keep surface
temperature at a life-supporting level over billions of years
despite a tremendous increase in solar energy output, and just the
wonder of the cosmos that is being revealed by our largest
telescopes, we come to realize the transcendent power and kindness
of our Creator.